the three months ended June 30, 2023 and 2022, the Company sold 54 and 58 systems, respectively, that were recognized as NeuroStar Advanced Therapy System capital revenue during each period. Additionally, for the three months ended June 30, 2022 the Company executed 1 operating lease agreement, that contributed to operating lease revenue.
U.S. treatment session revenue for the three months ended June 30, 2023 was $12.3 million, an increase of 9% compared to the three months ended June 30, 2022 revenue of $11.3 million. The revenue growth was primarily driven by an increase in treatment session volume over the prior year quarter due to increase in demand for treatment sessions.
Cost of Revenues and Gross Margin
Cost of revenues increased by $0.8 million, or 20%, from $4.0 million for the three months ended June 30, 2022 to $4.8 million for the three months ended June 30, 2023. Gross margin decreased from 75.3% for the three months ended June 30, 2022 to 72.5% for the three months ended June 30, 2023. The decline in gross margin was driven by the higher operational costs related to the contract manufacturer transition, as well as the software amortization expense from the latest product release.
Sales and Marketing Expenses
Sales and marketing expenses decreased by $2.1 million, or 16%, from $13.7 million for the three months ended June 30, 2022 to $11.6 million for the three months ended June 30, 2023. The decrease was primarily due to reduced spending in marketing on brand development. The new brand development initiative had completed in 2022. Additionally, in 2022 Neuronetics offered a retention program to sales personnel. This retention program was not continued in 2023, resulting in a decrease in sales personnel expense.
General and Administrative Expenses
General and administrative expenses decreased by $0.2 million or 2% from $6.4 million for the three months ended June 30, 2022, to $6.2 million for the three months ended June 30, 2023. The decrease was primarily due to decreased legal costs in the current period versus the prior year quarter.
Research and Development Expenses
Research and development expenses increased by $0.4 million, or 16%, from $2.0 million for the three months ended June 30, 2022 to $2.4 million for the three months ended June 30, 2023. The increase in research and development was primarily due higher clinical research and personnel expense as Neuronetics continues to look to increase the usability of the NeuroStar System.
Interest Expense
Interest expense increased by $0.1 million, or 14% from $1.0 million for the three months ended June 30, 2022 to $1.1 million for the three months ended June 30, 2023 due to increases in interest rate and debt balance.
Other Income, Net
Other income, net increased by $3.2 million from $0.4 million for the three months ended June 30, 2022 to $3.6 million for the three months ended June 30, 2023, primarily as a result of the ERC of $2.9 million and increased interest income earned on the Company’s money market accounts and notes receivable.