In patients with type II diabetes mellitus blood glucose control may gradually worsen despite diet and oral hypoglycaemic agents. We investigated which possible factors determine the decision for elderly type II diabetic patients to start with insulin therapy in this situation. According to Fishbein and Ajzen an attitude-behaviour model was constructed which included the determinants intention, attitude, subjective norm and self-efficacy. Fifty-three patients (age 68 +/- 9 years) participated in the study: 26 already used insulin for 7 months, 27 were tablet-treated but poorly-controlled (fasting blood glucose > 8.0 mmol/l and glycosylated haemoglobin > 8.0%). In both groups of patients, a high correlation (r = 0.81, P < 0.001) existed between the intention to and the actual start or continuation of insulin therapy. The main factor (70%) explaining the intention towards insulin treatment was the subjective norm: patients are guided by the opinion of important other persons, especially the treating internist, the family physician and the diabetes nurse. Tablet-treated patients reported a negative opinion of their physician towards their treatment with insulin. Intention to behaviour was also determined by self-efficacy, e.g. expectations about the skills concerning injecting insulin. Self-monitoring of blood glucose and knowledge about diabetes positively influenced self-efficacy, attitude towards and intention to use insulin. Education needs to enforce the subjective norm and improve the patient's self-efficacy. A stimulating attitude of the treating physician towards insulin therapy is essential.