The development of body mass index (BMI) was measured during the first 6 months of life in three groups of infants [human immunodeficiency virus (HIV) -uninfected, n = 92; later symptomatic HIV-infected, n = 18; early symptomatic HIV-infected, n = 9] born to HIV-positive mothers and compared with a reference group (n = 65) born to healthy mothers. A trend towards lower values in the two groups of HIV-infected infants was already evident at birth. Among the four groups, HIV-uninfected infants showed the highest BMI values while the early-infected infants showed the lowest BMI values at all measurements. The later-infected group had a value close to the reference at 1 month, and then increased at slower rates than the uninfected and the reference groups. Infants born to HIV-positive mothers may have higher energy and nutrient requirements after birth, either to sustain an increased BMI development (when uninfected) or to meet catabolic mechanisms (when infected).