OBJECTIVE To prospectively evaluate growth parameters assessed by weight and length in infected and uninfected infants born to HIV-1-infected mothers and followed from birth to 18 months. METHODS A cohort consisting of ninety-seven uninfected and forty-two infected infants born to HIV-infected mothers enrolled from 1995 to 2004, and admitted during their first 3 months of life at a referral Pediatric AIDS Clinic in Belo Horizonte, Brazil. Infants were followed until 18 months of age. Data were analysed using mixed-effects linear regression models for weight and length fitted by restricted maximum likelihood. RESULTS Infected infants contributed to 466 weight and 411 recumbent length measurements. Uninfected infants provided 924 weight and 907 length measurements. Mean birth weight and length were similar in both groups, 3.1 (sd 0.4) and 3.0 (sd 0.5) kg, and 48.7 (sd 1.4) and 48.8 (sd 2.9) cm for uninfected and infected infants, respectively. However, HIV-1 infection had an early impact in growth impairment: at 6 months of age, HIV-infected children were 1 kg lighter and 2 cm shorter than the uninfected. CONCLUSIONS Growth faltering in weight, but not length, in HIV-infected children in Brazil is more marked than that reported in a European cohort, probably reflecting background nutritional deficiencies and concomitant infections. In these settings, early and aggressive nutritional management in HIV-1-infected infants should be a priority intervention associated with the antiretroviral therapy.