Thirty-six middle schools' boys, 14 to 19 years of age are selected as subjects. Most of them have been previously found to show symptoms associated with riboflavin deficiency. Their riboflavin intake is calculated to be about 0.4 mg per person per day.A one-hour urine sample is collected at S a. m. Immediately after collection, the boys are subjected to a 2 mg riboflavin loading test administered orally. Hourly urine samples' are collected for the four hours directly following the intake.The subjects are divided into 4 groups with 8 to 10 students in each group. Each student in group I is given orally a daily riboflavin supplement of 0.5 mg; group II, 1.0 mg; group III, 1.5 mg; and group IV receives no riboflavin supplement, but ointment treatment for scrotal dermatitis is applied. The procedure is carried out for a period of 14 days. The boys eat the ordinary school food and participate in the usual school activities.At the end of the 14-day period, a 1-hour urine sample is again collected. Another 2 mg loading test is performed, hourly urine samples being collected for 4 hours. All the urine samples are preserved with glacial acetic acid and toluene and stored in an electric refrigerator. The Lactobacillus casei method is used for the analysis of riboflavin.The results of the one-hour urine riboflavin analysis agree well with the dietary survey, showing very low values before the supplement, averaging 1.4 micrograms for the hour. After the administration of the supplement for two weeks, the riboflavin content in the one-hour urine samples increases stepwise with the increase in supplement, the highest being 11.4 for the 1.5 mg supplemented group. No increase is observed in the group IV using ointments.The 4-hourly urinary riboflavin excretion following the administration of the 2 mg load averages over per cent before the supplement and increases to 11.5, 15.6 and 17.8 per cent for the 0.5, 1.0 and 1.5 mg supplemented groups respectively. The curves in Fig. 1 show the hourly excretion of the 3 groups before and after supplementation.Clinical observations show that supplementation of 1 mg or more relieves some of the deficiency symptoms.It is suggested that in addition to the daily intake of about 0.4 mg 時iboflavin in the diet, 1.5 mg more should be added to keep the middle school boys in optimal riboflavin nutrition.