The aim of the present study was to evaluate the role of residual insulin production in long-term Type 1 (insulin-dependent) diabetes mellitus. Ninety-seven patients with a disease duration of 9-16 years and onset before the age of 30 years were studied. C-peptide excretion in 24-h urine samples was measured as an indicator of residual insulin production. Thirty-five patients (36%) excreted C-peptide (greater than or equal to 0.2 nmol); as many as possible of them were carefully matched with a non-excretor patient with regard to age at onset of diabetes and disease duration. Twenty-nine pairs were obtained, and 22 of them agreed to participate in further investigations of glycaemic control and microangiopathic lesions. The patients who excreted C-peptide had significantly lower HbA1c than the non-excretor group, 6.9 +/- 0.3% vs 7.9 +/- 0.3%, (p less than 0.025). Moderate-to-advanced background retinopathy was found in 2 patients in the excretor group and in 7 patients in the non-excretor group. Microalbuminuria [ratio of albumin: creatinine (mg/l:mmol/l) greater than or equal to 5] was found in 1 and in 5 patients, respectively, while proteinuria [ratio of protein: creatinine (mg/l:mmol/l X 10) greater than or equal to 136] was found in 0 and in 4 patients, respectively. Microalbuminuria and/or proteinuria was found in 7 of the non-excretor group as compared to 1 in the excretor group (p = 0.046).(ABSTRACT TRUNCATED AT 250 WORDS)