The aim of the present cross-sectional study was to determine in patients who have had insulin-dependent diabetes mellitus for a long time, the relationship between residual insulin secretion and self-monitoring of blood glucose, psychological factors, and blood glucose control (HbA1c). In 97 patients with a disease duration of 9-16 years a sample from a 24-h urine collection was analysed for C-peptide excretion. In 35 of these patients an excretion greater than or equal to 0.2 nmol/24 h was detected. They were matched for age at onset and disease duration with patients without any detectable C-peptide excretion. Twenty-nine matched pairs were obtained and 22 of them have been further investigated. HbA1c was lower in the C-peptide excretors than in the non-excretors (6.9 +/- 0.3 vs 7.9 +/- 0.3%) (p less than 0.025). The patients were interviewed by means of a questionnaire dealing with their 'health beliefs'. It was shown that in the group with residual insulin secretion there was a significantly lower perceived risk of developing kidney disease (p less than 0.025) than in the group without residual insulin secretion, but in all other items measured there was no difference between the two groups. Furthermore, in the group with residual insulin secretion we found a correlation (p less than 0.01) of low HbA1c with high frequency of self-monitoring of blood glucose and high perceived vulnerability.(ABSTRACT TRUNCATED AT 250 WORDS)