of $0.1 million, or 0.6%, in sales of the airway clearance product line compared to the fourth quarter of 2022.
Gross profit in the fourth quarter of 2023 increased $3.9 million, or 7.5%, to $56.0 million, compared to $52.1 million in the fourth quarter of 2022. Gross margin was 72.1% of revenue, compared to 70.5% of revenue in the fourth quarter of 2022. Non-GAAP gross margin was 72.5% of revenue, compared to 71.2% of revenue in the fourth quarter of 2022.
Operating expenses in the fourth quarter of 2023 were $44.2 million, consistent with the fourth quarter of 2022.
Operating income was $11.8 million in the fourth quarter of 2023, compared to $7.9 million in the fourth quarter of 2022. Non-GAAP operating income in the fourth quarter of 2023 was $12.7 million, compared to $9.5 million in the fourth quarter of 2022.
Other expense was $36,000 in the fourth quarter of 2023, compared to $950,000 in the fourth quarter of 2022.
Income tax expense was $3.6 million in the fourth quarter of 2023, compared to $2.3 million in the fourth quarter of 2022.
Net income in the fourth quarter of 2023 was $8.2 million, or $0.35 per diluted share, compared to $4.6 million, or $0.23 per diluted share, in the fourth quarter of 2022. Non-GAAP net income in the fourth quarter of 2023 was $8.9 million, compared to $5.9 million in the fourth quarter of 2022.
Weighted average shares used to compute diluted net income per share were 23.8 million and 20.3 million for the fourth quarters of 2023 and 2022, respectively.
Adjusted EBITDA was $15.4 million in the fourth quarter of 2023, compared to $12.1 million in the fourth quarter of 2022.
Full Year 2023 Financial Results
Total revenue for the twelve months ended December 31, 2023, increased $27.6 million, or 11.2%, to $274.4 million, compared to $246.8 million for the twelve months ended December 31, 2022. The increase in revenue was attributable to an increase of $29.5 million, or 13.9%, in sales and rentals of the lymphedema product line, partially offset by a decrease of $1.8 million, or 5.3%, in sales of the airway clearance product line.
Net income for the twelve months ended December 31, 2023, was $28.5 million, or $1.23 per diluted share, compared to a net loss of $17.9 million, or $0.89 per diluted share, for the twelve months ended December 31, 2022. Non-GAAP net income for the twelve months ended December 31, 2023, was $29.5 million, compared to a non-GAAP net loss of $3.6 million for the twelve months ended December 31, 2022.
Weighted average shares used to compute diluted net income (loss) per share were 23.2 million and 20.1 million for the twelve months ended December 31, 2023 and 2022, respectively.