Background: Limited research has documented an association between soil-transmitted helminth (STH) infections and child development. This has recently been identified as an important knowledge gap. Methods: A longitudinal cohort study was conducted in Iquitos, Peru, between September 2011 and July 2016. A cohort of 880 children, recruited at 1year of age, was followed up to 5years. STH infection was measured annually and child development was measured with the Wechsler Preschool and Primary Scale of Intelligence III (WPPSI-III) at 5years. Linearregression models were used to investigate the effect of the number of detected STH infections between 1 and 5years of age on WPPSI-III scores at 5years of age. Bayesian latent class analysis was used to adjust for exposure misclassification. Results: A total of 781 (88.8%) children were included in the analysis. In multivariable analysis, adjusted for STH misclassification, increasing numbers of Ascaris, Trichuris, hookworm and any STH infections were associated with lower WPPSI-III scores. Among the largest observed effects were those for the effect of Ascaris infection on verbal IQ scores [difference in IQ (95% CrI) for two, three, and four or five detected infections compared with zero or one infection:-8.27 (-13.85,-3.10),-6.69 (-12.05,-2.05) and-5.06 (-10.75, 0.05), respectively]. Misclassification of STH infection generally led to a bias towards the null. Conclusions: These results document an association between STH infection and child development. The results highlight the importance of adjusting for STH misclassification; however, future research is needed to accurately determine the sensitivity of STH diagnostic techniques. STH control in preschool children may contribute to lowering the disease burden associated with poor child development.