Abstract: We assessed laboratory monitoring after combination antiretroviral therapy initiation among 3678 patients in a large US multisite clinical cohort, censoring participants at last clinic visit, combination antiretroviral therapy change, or 3 years. Median days (interquartile range) to first hematologic, hepatic, renal, and lipid tests were 30 (18–53), 31 (19–56), 33 (20–59), and 350 (96–1106), respectively. At 1 year, approximately 80% received more than 2 hematologic, hepatic, and renal tests consistent with guidelines. However, only 40% received 1 or more lipid tests. Monitoring was more frequent in specific subgroups, likely reflecting better clinic attendance or clinician perception of higher susceptibility to toxicities.