I. The urinary hydroxyproline : creatinine index was measured in forty-one preschool children, ranging in age from IS months to 8 years, before and after they had received a supplement of 10 mg riboflavin/d for 7 d. It was found that the supplement increased the index in twenty-scven of the children. 2. The children were divided into two groups, those with indices C 1.5 and those with indices > I' 5 before supplementation. 3. The hydroxyproline :creatinine index was found to increase significantly (P 1.5. 4. There was no significant difference between the two groups, in either riboflavin status before supplementation or mean age. 5. The part played by riboflavin in protein utilization and growth is discussed. The urinary hydroxyproline : creatinine index was introduced by Whitehead (1965) as a criterion for USC in the early detection of marginal malnutrition. It was suggested that index values below 2-0 showed marginal malnutrition and Whitehead (1965) found that subjects with clinically recognizable signs of malnutrition usually had index values below 1.5. Studies carried out in two villages in north-east Thailand in 1969 showed that index values from below 1.0 to above 5.0 were found for the infants and preschool children in both villages but not one of these children was clinically malnourished (Migasena, Thurnham, Pongpaew, Jintakanon & Harinasuta, 1971). Subclinical malnutrition, however, was possibly present among the village children since their growth rates were below those of a group of well-nourished Thai children and Western standards (Migasena et al. 1971). In addition, evidence of inadequate dietary intakes in civilian diets were revealed by a nutritional survey carried out by the Interdepartmental Committee on Nutrition for National Defense (1962) which showed that, although protein and caloric intakes were probably adequate, vitamin intakes were generally low. Biochemical ariboflavinosis has been demonstrated among the preschool children in both villages referred to above (Thurnham, Migasena & Pavapootanon, 1970; Thurnham, Migasena & Vudhivai, 1971). A low dictary intake of riboflavin has been shown to reduce the growth rates of weanling rats on both high-protein and lowprotein diets (Sugioka, Porta, Corey & Hartroft, 1969). Thus, it was of interest to