In order to establish the human requirements of niacin, it is first important to know how much tryptophan is converted to niacin in the human body. In a general, 60 mg of tryptophan is equivalent to 1 mg of niacin, whereas the conversion ratio of tryptophan to niacin is yet to be confirmed. The aim of this study was to know the conversion ratio of tryptophan to niacin in Japanese females fed a purified diet, which followed the Japanese Dietary Reference Intakes. Ten young Japanese females were housed in the same facility and given the same daily living activity schedule for 7 d. The composition of their purified diet was conformed to the Dietary Reference Intakes in Japan. The diet was niacin free. In order to investigate the conversion ratio, daily urinary outputs were collected. Tryptophan-niacin metabolites in the urine were measured and the conversion ratio of tryptophan to niacin calculated. The conversion ratio was calculated by comparing the dietary intake of tryptophan and the sum of the niacin catabolites such as N1-methylnicotinamide, N1-methyl-2-pyridone-5-carboxamide, and N1-methyl-4-pyridone-3-carboxamide, which were derived only from the dietary intake of tryptophan. The ratio was calculated as 1.5 +/- 0.1 (mean +/-SE for 10 women; in molar basis) on the last day of the experiment. It was calculated that if the excretory percentage of niacin metabolites in the urine were 60%, of the tryptophan ingested, the conversion factor would be a value of 67, meaning that is 67 mg of tryptophan is equal to 1 mg of niacin.