Opinions diverge as to the frequency of hypoglycaemia in patients under continuous subcutaneous insulin infusion (CSII). In this prospective study the incidence and severity of hypoglycaemia were evaluated in 10 type I brittle diabetic patients under conventional treatment (period A), then under chronic treatment with CSII for at least 1 year (period B: the first 3 months; period C: the last 3 months). Metabolic control (as assessed from mean blood glucose, glycaemia lability indices and glycosylated haemoglobin A1) significantly improved between periods A and B (p less than 0.01). The occurrence of biochemical hypoglycaemia (less than 3 mmol/l) was reduced by 52% and that of clinical hypoglycaemia by 60%. The results observed in period C were not different from those observed in period B. One hypoglycaemic coma occurred under CSII (as opposed to 4 during period A); it was secondary to reduction in food intake and had no sequelae. Thus, in this study, the improvement in metabolic control was associated with a reduction in the frequency and severity of hypoglycaemia. We consider that patients presenting with frequent and severe attacks of hypoglycaemia under conventional insulin treatment can and should benefit from CSII.