The C-peptide/creatinine (Cr) ratio in early morning urine was evaluated to assess B-cell function. The subjects were 12 boys and 36 girls with insulin-dependent diabetes mellitus (IDDM). The controls were 130 boys and 137 girls aged 4-15 years. There was a significant inverse correlation of this ratio with the duration of insulin therapy (r = -0.5807, P less than 0.01). The daily insulin dose in U/kg was significantly different among the following groups: 1.22 +/- 0.31 U/kg in group 1 with undetectable C-peptide, 0.94 +/- 0.37 in group 2 with a decreased ratio and 0.45 +/- 0.28 in group 3 with a normal ratio. HbA1 levels were 11.3 +/- 1.6% in group 1 and 9.2 +/- 1.1% in group 3. The difference was significant. The result shows that the C-peptide/Cr ratio in early morning urine is useful for assessing B-cell function in diabetic children.