Subclinical mastitis is common in HIV-infected women and is a risk factor for mother-to-child transmission of HIV. The purpose of this study was to examine the effect of vitamin supplementation [vitamin A + β-carotene, multivitamins (B complex, C, and E), or multivitamins, including vitamin A + β-carotene] on the risk of subclinical mastitis during the first 2 y postpartum among HIV-infected women. The study was a randomized, placebo-controlled, clinical trial including 674 HIV-infected, antiretroviral naïve Tanzanian women who were recruited during pregnancy and followed-up after delivery. Breast milk samples were obtained approximately every 3 mo. Any subclinical mastitis was defined as a ratio of the sodium to potassium (Na:K) breast milk concentrations > 0.6 and further classified as either moderate (Na:K ≥ 0.6 and ≤ 1) or severe (Na:K > 1.0). Fifty-eight percent of women had at least 1 episode of any subclinical mastitis. Women assigned to multivitamins (B complex, C, and E) had a 33% greater risk of any subclinical mastitis (P = 0.005) and a 75% greater risk of severe subclinical mastitis (P = 0.0006) than women who received the placebo. Vitamin A + β-carotene also increased the risk of severe subclinical mastitis by 45% (P = 0.03). Among women with CD4+ T-cell counts ≥ 350 cells/μL, multivitamin intake resulted in a 49% increased risk of any subclinical mastitis (P = 0.006); by contrast, there were no treatment effects among women with CD4+ T-cell counts < 350 cells/μL (P- interaction for treatment × CD4+ T-cell count = 0.10). Supplementation of HIV-infected women with vitamins increased the risk of subclinical mastitis.