OBJECTIVE To investigate whether patients require insulin as a result of poor adherence to oral hypoglycaemic agents (OHAs) in Type 2 diabetes. METHODS A diabetes information system and a database of drugs dispensed for the study period 1993-1996 were used in Tayside, Scotland (population 400 000). Patients aged over 34 years with Type 2 diabetes who had at least 6 months exclusive therapy with OHAs (sulphonylureas or metformin) prior to insulin treatment were identified. Intended duration of every OHA prescription was calculated from prescription details. Adherence was estimated by dividing total intended duration of OHA therapy by study time for each patient, and compared between those who did and did not convert to insulin. RESULTS There were 2537 patients on sulphonylureas (51% male, mean age 67 years). There was improved adherence in the 262 patients who commenced insulin, who had mean adherence of 88.3% (95% confidence interval (CI) 85.9-90.6%) compared with the remaining 2275 patients whose mean adherence was 87.4% (95% CI 86.7-88.2%). In a logistic regression analysis, the adjusted odds ratio (OR) for commencing insulin was 1.20 (95% CI 1.07-1.35) for a quartile increase in adherence. There were 1519 patients on metformin (49% male, mean age 64 years). Mean adherence was 79.7% (95% CI 76.4-83.1%) and 83.1% (95% CI 82.0-84.1%) in 169 patients who did and 1350 who did not commence insulin, respectively, with an adjusted OR for a quartile increase in adherence of 0.91 (95% CI 0.78-1.07). CONCLUSIONS Despite suboptimal adherence to OHAs in Type 2 diabetes, this is not associated with subsequent requirement for insulin.