by $2.6 million from the three months ended March 31, 2017 compared to the three months ended March 31, 2018 due to higher unit sales of both NeuroStar Advanced Therapy Systems and treatment sessions. International revenues increased slightly for the three months ended March 31, 2018 compared to the three months ended March 31, 2017, primarily due to the recognition of deferred revenue related to the upfront and milestone payments received in 2017 under our distribution agreement with Teijin for Japan.
Revenues in the United States increased by $2.6 million, or 35%, from $7.4 million for the three months ended March 31, 2017 to $10.0 million for the three months ended March 31, 2018. NeuroStar Advanced Therapy System revenues in the United States grew by $1.1 million, or 80%, in the three months ended March 31, 2018 compared to the three months ended March 31, 2017, based on 74% higher unit volume and 6% higher average selling prices. NeuroStar Advanced Therapy System revenues represented 18% and 24% of U.S. revenues for the three months ended March 31, 2017 and 2018, respectively. We believe the growth in NeuroStar Advanced Therapy System revenues between these two periods was the result of the efforts of our expanded commercial organization and increased marketing efforts. As of March 31, 2018, we had an installed base of 781 active systems in the United States, compared to 671 as of March 31, 2017.
Treatment session revenues in the United States represented 78% and 72% of total revenues in the United States for the three months ended March 31, 2017 and 2018, respectively, and increased by 26% from the three months ended March 31, 2017 to the three months ended March 31, 2018. The increase in United States treatment session revenues was primarily the result of a 30% increase in the number of treatment sessions performed. This was offset by a 4% decline in average selling price due to certain volume pricing discounts within our existing customer base.
Cost of Revenues and Gross Margin
Cost of revenues increased by $0.9 million, or 60%, from $1.5 million for the three months ended March 31, 2017 to $2.5 million for the three months ended March 31, 2018. The increase was primarily due to increased NeuroStar Advanced Therapy System sales becoming a larger portion of the sales mix. Gross margin decreased from 80% for the three months ended March 31, 2017 to 76% for the three months ended March 31, 2018. The decrease was due to the higher mix of NeuroStar Advanced Therapy System revenues in the three months ended March 31, 2018 in relation to treatment session revenues, as well as the reduction in the average selling price of our treatment sessions contributed to the decline in gross margin.
Sales and Marketing Expenses
Sales and marketing expenses increased by $1.8 million, or 29%, from $6.3 million for the three months ended March 31, 2017 to $8.1 million for the three months ended March 31, 2018. The increase was primarily due to increased personnel costs as a result of our sales and marketing expansion activities, as well as higher marketing expenses and sales commission costs, consistent with our growth in revenues.
General and Administrative Expenses
General and administrative expenses increased by $1.0 million, or 61%, from $1.6 million for the three months ended March 31, 2017 to $2.6 million for the three months ended March 31, 2018. The increase was primarily due to an increase in personnel and legal, accounting and other professional services expenses required to support the growth of our business and to ready the infrastructure for public company reporting.
Research and Development Expenses
Research and development expenses decreased $0.5 million, or 23%, from $2.0 million for the three months ended March 31, 2017 to $1.6 million for the three months ended March 31, 2018. The decrease was primarily due to declines in spending relating to our adolescent study, which were partially offset by expenses relating to the launch of the next generation of our NeuroStar Advanced Therapy System and TrakStar practice management system.
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