Knowledge about diabetes was assessed using a previously described interactive computer-based questionnaire in 79 patients with insulin-dependent (IDDM) and 72 with non-insulin-dependent (NIDDM) diabetes mellitus routinely attending a single diabetic clinic. Simple linear correlation of total knowledge score with glycosylated haemoglobin (HbA1c) showed no significant relationship for either IDDM (r = 0.12: p = 0.18) or NIDDM (r = 0.15: p = 0.1). However, quintile grouping of knowledge scores showed the mean HbA1c to be significantly higher in the lowest scoring NIDDM quintile (10.6 +/- 0.5: +/- SE) with respect to the pooled mean of all the higher scoring quintiles (9.0 +/- 0.3) (p = 0.027). Mean HbA1c (9.6 +/- 0.5) was also higher in the least knowledgeable IDDM quintile than any other quintile group (range 8.8-9.0) but this was not significant with respect to the pooled mean of higher scoring patients (p greater than 0.1). The mean age of the lowest scoring IDDM quintile group (60.5 +/- 13.9 years) was significantly higher (p less than 0.01) than higher scoring IDDM groups (mean age range 36.5-43.3 years) but age was not significantly related to HbA1c in IDDM subjects. IDDM showed greater knowledge of diabetes than NIDDM but ignorance in key areas was unacceptably high in both diabetic subtypes, indicating that regular knowledge assessment and educational reinforcement may be essential for good diabetic control as well as patient safety, particularly in older IDDM patients.(ABSTRACT TRUNCATED AT 250 WORDS)