Objective: Most Helicobacter pylori infections occur during childhood, but the health effects of childhood infection are poorly understood. We investigated whether growth decreases in the 2 months after acute H. pylori seroconversion. Methods: We performed a nested case-control study among children 6 months to 12 years of age in a community on the outskirts of Lima, Peru. Health interviews were completed daily. Anthropometric measurements were taken monthly. Sera were collected every 4 months and tested for H. pylori immunoglobulin G. Two-month height and weight gains of seroconverters were compared with gains of sex, age, and size-matched seronegative controls. Results: In the 2 months after H. pylori infection, 26 seroconverters gained a median of 24% less weight than 26 matched controls (interquartile range, 63% less to 21% more). In multivariate analysis, H. pylori infection attenuated weight gain only among children aged 2 years or older. This decrease was not explained by increased diarrhea. Conclusions: H. pylori seroconversion is associated with a slowing of weight gain in children aged 2 years or older. Reasons for this finding merit additional study. © 2002 Lippincott Williams & Wilkins, Inc.