Undernutrition is a frequent manifestation and an important health problem. The aim of this study was to analyse the variables of nutritional status among primary school children and to determine the risk factors associated with it. The study group was composed of 1576 students between 6 and 16 years of age. The prevalence of stunting, underweight, wasting were calculated according to WHO recommended cut-off points to define undernutrition. Chi-square tests, logistic regression analysis were used to investigate the relationship between the prevalence of undernutrition and the sociodemographic factors. The overall prevalence of stunting, underweight and wasting were found as 5.7, 4.6 and 1.0%, respectively. There were significant relationships between underweight and age, sex, number of family members, monthly family income. But, a significant correlation was found only between stunting and age. No correlation was found between wasting and sociodemographic factors. Undernutrition is still a common problem among primary school children in Istanbul. Identification of risk factors is essential for prevention of undernutrition.