An adolescent population ranging in age from 13 to 19 years and of a low socioeconomic status in New York City was surveyed for riboflavin deficiency. Deficiency was determined from estimation of erythrocyte glutathione reductase activity, an accurate reflector of riboflavin nutritional status. The overall prevalence of deficiency among those not on vitamin supplements was 26.6%. The prevalence did not depend on sex or age. There was a correlation between milk consumption and riboflavin nutritional status. The prevalence was highest among those consuming less than 1 cup/week and least among those taking 3 or more cups a day. The latter group was comparable in this respect to those receiving daily vitamin supplements. Adolescents are at a high risk for nutritional deficiencies because of their notoriously poor dietary habits, and the estimation of riboflavin deficiency may be an indicator of overall nutritional status.