The excretion of iodine both in a 24 hour urine collection and per g of creatinine in a single afternoon specimen was studied in 33 physicians and 29 nurses in Oslo. The mean 24 hour excretion in males was 216 μg/24 h and in females 165 μg/24 h. The mean ratio between total urinary iodine excretion in 24 hours and the excretion per g of creatinine in the afternoon specimen was found to be 1.97 (sd = 0.53) in males and 1.30 (sd = 0.49) in females. These data make it possible to calculate the mean 24 hour urinary iodine excretion in population studies on the basis of afternoon urine specimens, and also to estimate the accuracy of this calculation. The method was used on samples of urine from 210 healthy males and 73 healthy females in Oslo (mostly industrial workers). The estimate of the mean 24 hour iodine excretion in the males was 248 μg/24 h (sem 4.5), and in the females 173 μg/24 h (sem 8).