Background: Intestinal worm infections transmitted through the soil are the most common infection among parasitic infections. The key to worm eradication is to improve personal hygiene and environmental sanitation. This study aimed to identify several risk factors related to occurrence of Soil Transmitted Helminth (STH) infections in elementary school.Methods: A cross sectional study with purposive sampling method was carried out in a primary school children in a area of a Yogyakarta health Center from October to December 2009. Stool was examined by using the Kato Katz method and pupils were interviewed by questionnaires.Results: Two hundred and eleven subjects participated in this study, and 52 subjects (24.6%) had STH infection. The most frequent STH infection was Trichuris trichiura, and the least was mixed infection (Ascaris lumbricoides, Trichuris trichiura and Hookworm). The highest risk (2.8-fold) occurred among those with a habit of fingernail biting compared to those who did not bite fingernails [adjusted relative risk (RRa) = 2.80; 95% confidence interval (CI) = 1.22-4.04]. No hand washing before meals as well as no hand washing with soap after passing stool also increased the risk of STH infection by 2.2-fold.Conclusion: Fingernail biting and no hand washing before meals as well as no hand washing with soap after passing stool increased the risk of STH infections. (Health Science Indones 2011;2:81-6).