A total of 1078 human immunodeficiency virus (HIV) type 1-infected women from Tanzania were randomized in a placebo-controlled trial using a factorial design to examine the effects of supplementation with vitamin A (preformed vitamin A and beta carotene) and/or multivitamins (vitamins B, C, and E). Supplements were given during pregnancy and lactation. Children of women in the multivitamin arms had a significantly lower risk of diarrhea than did those in the no-multivitamin arm (P=.03). The mean CD4+ cell count was 151 cells/microL higher among children in the multivitamin arms than among those in the no-multivitamin arm (P=.0006). HIV-positive children experienced a benefit apparently similar to that in HIV-negative children (P=.34, by test for interaction). Maternal receipt of vitamin A significantly reduced the risk that the child would have cough with a rapid respiratory rate, a proxy for pneumonia (P=.03), but receipt of vitamin A had no effect on diarrhea or CD4+ cell count. Provision of multivitamin supplements (including those with vitamins B, C, and E) to HIV-infected, lactating women may be a low-cost intervention to improve their children's health.