This article tests the hypothesis that the presence of gastrointestinal parasites in Colombian boys is negatively associated with anthropometric characteristics, physical work capacity, blood hemoglobin (Hb) levels, and nutritional status. Anthropometric, Hb, &Vdot;O(2) max, and parasite load data were collected on 1,016 boys in Cali, Colombia. The boys were classified as lower socioeconomic class (SEC) from either urban or rural environments, and upper SEC from an urban environment. Sixty-three percent of the boys were infected with gastrointestinal parasites and, of the infected boys, 80-95% had light parasite loads. Parasites found included Necator americanus, Ascaris lumbricoides, Entamoeba histolytica, Trichuris trichiura, Giardia spp., and Enterobius vermicularis. Infected boys had significantly lower weight, stature, weight-for-height (among 6-9-year-old boys), Hb levels, and &Vdot;O(2) max (ANCOVA, controlling for age and SEC). In terms of nutritional status, infected boys were 1.47 times more likely to be classified as iron deficient than noninfected boys (chi-square, P < 0.001), and 1.61 times more likely to be classified as stunted (P < 0.001). Infection was not associated with wasting in any SEC group. In conclusion, light to moderate gastrointestinal parasite loads were associated with significantly lower weight, stature, weight-for-height (in 6-9-year-old boys), Hb levels, and &Vdot;O(2) max, and a significantly higher frequency of IDA and stunting. These data suggest that comprehensive analyses of the nutritional status of populations in regions endemic for parasitic infection should include testing for the presence of infection. Am. J. Hum. Biol. 11:763-771, 1999. Copyright 1999 Wiley-Liss, Inc.