Revenue increased by $3.0 million, or 102%, to $6.0 million for the six months ended June 30, 2021, compared to the six months ended June 30, 2020.
Revenue generated in the U.S. was $3.7 million for the six months ended June 30, 2021, an increase of $3.1 million, or 513%, over the six months ended June 30, 2020. Total HF revenue units in the U.S. totaled 111 and seven for the six months ended June 30, 2021 and 2020, respectively. HF revenue in the U.S. totaled $3.3 million and $0.2 million for the six months ended June 30, 2021 and 2020, respectively. The increase was driven by the continued growth following the commercial launch in 2020, which resulted in the expansion into new sales territories and increased physician and patient awareness of our BAROSTIM NEO. The number of sales territories in the U.S. increased by two to a total of eight during the six months ended June 30, 2021. Legacy hypertension revenue in the U.S. totaled $0.5 million and $0.3 million for the six months ended June 30, 2021 and 2020, respectively.
Revenue generated in Europe was $2.3 million for the six months ended June 30, 2021, a decrease of $96,000, or 4%, over the six months ended June 30, 2020. Total revenue units in Europe decreased to 99 from 106 for the six months ended June 30, 2021 and 2020, respectively. The revenue decrease was primarily due to the impact of the COVID-19 pandemic, which was partially offset by an increase in our average selling price. The number of sales territories in Europe remained consistent at six during the six months ended June 30, 2021.
Cost of goods sold increased $1.0 million, or 129%, to $1.8 million for the six months ended June 30, 2021, compared to the six months ended June 30, 2020. This increase was primarily due to higher sales of our BAROSTIM NEO.
Gross margin decreased to 70% for the six months ended June 30, 2021, compared to 74% for the six months ended June 30, 2020. Gross margin for the six months ended June 30, 2021 was lower due to a larger percentage of our revenue units coming from full systems, which require an IPG and a stimulation lead, as compared to individual IPG sales. This was partially offset by an increase in the average selling price.
Research and development expenses
R&D expenses decreased $0.4 million, or 9%, to $4.0 million for the six months ended June 30, 2021, compared to the six months ended June 30, 2020. This was driven by a $0.9 million decline in clinical study expenses, partially offset by a $0.5 million increase in consulting expenses and a $0.2 million increase in non-cash stock-based compensation expense.
Selling, general and administrative expenses
SG&A expenses increased $6.0 million, or 144%, to $10.1 million for the six months ended June 30, 2021, compared to the six months ended June 30, 2020. This was driven by an increase of $3.2 million in compensation expenses, including salaries and commissions, and other employee-related expenses, mainly as a result of increased headcount, a $1.0 million increase in marketing and advertising expenses, primarily related to the commercial launch of our BAROSTIM NEO in the U.S., a $0.7 million increase in consulting expenses, a $0.4 million increase in non-cash stock-based compensation expense, and $0.3 million of additional travel expenses.
Interest expense
Interest expense remained consistent at $1.2 million for the six months ended June 30, 2021 and 2020.