| ● | Increase in interest expense related to an increase in our outstanding debt under our credit facility |
| ● | Increase in tax expense due to improved results of operations |
High repeat business and client concentration are common in our industry. During the three months ended September 30, 2019 and 2018, 98% and 88%, respectively, of our revenue was derived from clients who had been using our services for more than one year, including clients acquired from eTouch Systems Corp. in March 2018. During the six months ended September 30, 2019 and 2018, 98% and 89%, respectively, of our revenue was derived from clients who had been using our services for more than one year, including clients acquired from eTouch Systems Corp. in March 2018. Accordingly, our global account management and service delivery teams focus on expanding client relationships and converting new engagements to long-term relationships to generate repeat revenue and expand revenue streams from existing clients. We also have a dedicated business development team focused on generating engagements with new clients to continue to expand our client base and, over time, reduce client concentration.
We derive our revenue from two types of service offerings: application outsourcing, which is recurring in nature; and consulting, including technology implementation, which is non-recurring in nature. For the three months ended September 30, 2019, our application outsourcing and consulting revenue represented 55% and 45%, respectively of our total revenue as compared to 53% and 47%, respectively, for the three months ended September 30, 2018. For the six months ended September 30, 2019, our application outsourcing and consulting revenue represented 56% and 44%, respectively, of our total revenue as compared to 53% and 47%, respectively, for the six months ended September 30, 2018.
In the three months ended September 30, 2019, our North America revenue increased by 11.0%, or $24.0 million, to $242.3 million, or 73.8% of total revenue, from $218.3 million, or 71.5% of total revenue, in the three months ended September 30, 2018. In the six months ended September 30, 2019, our North America revenue increased by 10.5%, or $44.9 million, to $472.8 million, or 73.0% of total revenue, from $427.9 million, or 70.7% of total revenue in the six months ended September 30, 2018. The increase in North America revenue for the three and six months ended September 30, 2019 was primarily due to the increase in revenue from clients in the C&T industry group, including customer contracts with certain existing customers acquired from third parties.
In the three months ended September 30, 2019, our European revenue decreased by 5.6%, or $3.4 million, to $57.0 million, or 17.4% of total revenue, from $60.4 million, or 19.8% of total revenue in the three months ended September 30, 2018. In the six months ended September 30, 2019, our European revenue decreased by 5.5%, or $7.0 million, to $120.1 million, or 18.5% of total revenue, from $127.1 million, or 21.0% of total revenue in the six months ended September 30, 2018. The decrease in European revenue for the three and six months ended September 30, 2019 was primarily due to a decline in revenue from one of our large banking clients and the substantial depreciation in the GBP against the U.S. dollar.
Our gross profit increased by $0.7 million to $89.9 million for the three months ended September 30, 2019, as compared to $89.2 million for the three months ended September 30, 2018. Our gross profit increased by $1.5 million to $174.2 million for the six months ended September 30, 2019 as compared to $172.7 million in the six months ended September 30, 2018. The increase in gross profit during the three and six months ended September 30, 2019, as compared to the three and six months ended September 30, 2018, was primarily due to higher revenue partially offset by higher onsite effort and subcontractor costs. As a percentage of revenue, gross margin was 27.4% and 29.2% in the three months ended September 30, 2019 and 2018, respectively. During the six months ended September 30, 2019 and 2018, gross margin, as a percentage of revenue, was 26.9% and 28.5%, respectively. The decrease in gross margin during three and six months ended September 30, 2019 was primarily due to higher onsite effort, an increase in subcontractors cost and lower utilization.
We perform our services under both time-and-materials and fixed-price contracts. Revenue from fixed-price contracts represented 39% and 40% of total revenue, and revenue from time-and-materials contracts represented 61% and 60% of total revenue for the three months ended September 30, 2019 and 2018, respectively. Revenue from fixed-price contracts represented 40% and 40% of total revenue and revenue from time-and-materials contracts represented 60% and