C-peptide immunoreactivity (CPR) in 24-h urine was assayed in 13 diabetic patients during and after recovery from ketoacidosis. In 10 patients who remained insulin-dependent on discharge and in the subsequent follow-up period, urine CPR was low (18 μg/day or less), while in three patients who were ultimately controlled by diet or sulfonylureas, urine CPR was normal (59–92 μg/day, normal value 74 ± 26 μg/day). In the latter group, urine CPR in one patient assayed during ketoacidosis was 6 μg/day and, in another patient, it was 22 μg/day on the 3rd day of the admission. This may imply that in the latter group, B-cell function was decompensated in ketoacidosis, but was restored after recovery. Clinical courses suggested that these patients were not in the remission phase of IDDM, but belonged to NIDDM. Among other groups of diabetic patients, urine CPR in those treated with diet or sulfonylureas was 72 ± 30 μg/day and always higher than 20 μg/day. The prevalence of urine CPR less than 20 μg/day was more frequent in those with younger onset of diabetes, higher insulin dosage, unstable diabetes, or previous history of ketoacidosis among insulin-treated patients. We suggest that urine CPR less than 20 μg/day is an index of insulin dependency, although in a state of extreme decompensation of B-cells such as in ketoacidosis, urine CPR can be decreased low even in NIDDM.