changes in our effective tax rate, which was attributable to a change in projected taxable income as compared to the same period last year.
Net loss in the third quarter of 2021 was $3.4 million, or $0.17 per diluted share, compared to net income of $2.4 million, or $0.12 per diluted share, in the third quarter of 2020. Non-GAAP net loss in the third quarter of 2021 was $1.6 million, compared to Non-GAAP net income of $3.0 million in the third quarter of 2020.
Weighted average shares used to compute diluted net income/loss per share were 19.8 million and 19.7 million in the third quarters of 2021 and 2020, respectively.
Adjusted EBITDA was $4.1 million in the third quarter of 2021, compared to $6.2 million in the third quarter of 2020.
First Nine Months 2021 Financial Results:
Total revenue for the nine months ended September 30, 2021, increased $18.4 million, or 14%, to $146.3 million, compared to $127.9 million for the nine months ended September 30, 2020. The increase in revenue was driven by an increase of $13.9 million, or 12%, in sales and rentals of the Flexitouch system, an increase of $3.7 million, or 24%, in sales and rentals of the Entre system and $0.9 million in sales of the recently acquired AffloVest respiratory therapy business for the nine months ended September 30, 2021. Revenue for the nine months ended September 30, 2021, benefited from the initial stages of recovery from the COVID-19 pandemic through the second quarter of 2021, as well as an expanded prescriber base. However, in the third quarter of 2021 revenue was negatively impacted by the prolonged recovery from COVID-19, including the resurgence due to the Delta variant during the period, which resulted in restricted access to clinics and hospitals and disrupted the recovery in patient visits versus the pre-COVID environment. In addition, the challenging labor market impacted our ability to recruit and retain quality candidates for our direct sales force.
Net loss for the nine months ended September 30, 2021, was $4.3 million, or $0.22 per diluted share, compared to a net loss of $12.7 million, or $0.66 per diluted share, for the nine months ended September 30, 2020. Non-GAAP net loss for the nine months ended September 30, 2021, was $1.1 million, compared to Non-GAAP net loss of $8.5 million for the nine months ended September 30, 2020.
Weighted average shares used to compute diluted net income/loss per share were 20.0 million and 19.3 million for the nine months ended September 30, 2021 and 2020, respectively.
Adjusted EBITDA was $8.2 million in the nine months ended September 30, 2021, compared to $5.2 million in the nine months ended September 30, 2020.
Balance Sheet Summary
On September 8, 2021, the Company entered into an amendment to its credit agreement that, among other things, adds a $30.0 million incremental term loan to the $25.0 million revolving credit facility provided by the credit agreement. The term loan and the revolving credit facility mature on September 8, 2024.