We analyzed 369 patients with no prior Mycobacterium avium complex (MAC) infection and CD4 <50 cells/μL (baseline), while on combination antiretroviral therapy(cART), for incidence rates of primary MAC infection during the 6 months after baseline, by prophylaxis status. Of participants (median age, 40 years old), most were male (81%) and about half were non-white; at baseline, 81% of participants were on cART >60 days and 19% had HIV RNA <1000 copies/mL, whereas 65% had HIV RNA >10,000 copies/mL. Eleven patients had MAC infection within 6 months baseline (rate=0.6/100 person months): 4/175 on MAC prophylaxis vs. 7/194, no MAC prophylaxis (p=0.64). Of the 11 patients, seven had HIV RNA >10,000, and three >1000-9999 copies/mL at baseline (one missing). Median time to MAC infection was 62 days (IQR 43-126, maximum 139 days). No MAC infection occurred among 71 (19%) patients virologically suppressed (HIV RNA <1000 copies/mL) at baseline, including 41 patients with no MAC prophylaxis during follow-up. A small number of eligible virologically suppressed participants and the lack of data on cART/MAC prophylaxis adherence limited our observational nonrandomized study. Primary MAC prophylaxis may not be required for cART-virologically suppressed patients with CD4 <50 cells/mL.