The aim of this study was to prospectively follow a cohort of children without Helicobacter pylori infection and to compare growth velocity in the children who become infected during follow-up with that of children who remained infection-free.Three hundred forty-seven children in general good health, aged 12 to 60 months, who tested negative for H. pylori by the 13C-urea breath test, from three daycare centers in a lower-middle class borough of Cali, Colombia, were monitored for 2.5 years. Anthropometric measurements were performed every 2 months and breath tests every 4 months. Linear mixed models were used to analyze growth velocity in relation to onset of H. pylori infection.One hundred five (30.3%) children who were uninfected at the start of the study became infected during follow-up. Growth velocity in infected children was reduced by 0.042 +/- 0.014 cm/mo (P = 0.003) (approximately 0.5 cm/yr) after adjusting for age. The rate of deceleration in growth velocity was relatively constant over time.Among these lower-middle class children aged 12 to 60 months from a population with high prevalence of H. pylori infection, a new and sustained infection was followed by significant growth retardation.