Despite the common coexistence of vitamin A deficiency and Ascaris infection in preschool children in developing countries, and despite the widespread use of vitamin A supplements, remarkably little is understood about the impact of vitamin A supplementation on this gastrointestinal nematode. The Ministry of Health of Panama recently initiated a vitamin A supplementation program in rural indigenous populations. We took advantage of this initiative to assess the benefit of 200,000 IU (60 mg retinol) vitamin A on reinfection with Ascaris following deworming. Baseline stool exams, anthropometry, and socio-economic data were collected for 328 preschool children from 12-60 mo of age (106 supplemented within previous 3 mo and 222 unsupplemented within previous 6 mo). All children were dewormed with albendazole, and reinfection levels were monitored 3 and 5 mo later. Baseline prevalence of Ascaris was 79.5%. Stepwise regression showed that Ascaris intensity was lower in Vit A-supplemented children at baseline and 3 mo after deworming, but not after 5 mo. As 61% of the children were stunted, the impact of supplementation on Ascaris reinfection was examined separately for stunted and children of normal height. Prevalence and intensity of Ascaris at baseline and 3 mo after deworming were lower in children of normal height, but in stunted children the benefit was restricted to those who were dewormed within 6 wk of supplementation. Our study provides evidence that combined vitamin A supplementation and deworming reduces Ascaris reinfection in children living in areas of chronic parasitosis, but that the duration of the benefit is less in stunted children.