Overnight metabolic studies in 39 poorly controlled insulin-treated diabetic patients aged 9 to 66 years showed hypoglycaemia (blood-glucose less than 2 mmol/1) in 22 patients; it lasted 3 h or more in 17. Hypoglycaemic symptoms were very mild or absent, but 19 patients had other features of overtreatment with insulin. These included lethargy, depression, night sweats, morning headaches, fits (3 patients), glycogen-laden hepatomegaly (3), and acquired tolerance to high doses of insulin (mean 1 u/kg/24 h). The best clinical clue to recurrent nocturnal hypoglycaemia was the intermittent occurrence of symptoms, however "mild" and infrequent these appeared to be. Reduction of insulin by a mean of 25% in these patients (without change of species) did not result in loss of overall control; 1 patient with recurrent ketoacidosis was stablished on 40% of his initial dose. It is difficult, sometimes impossible, to achieve good overnight control with conventional once or twice daily insulin therapy. Since patients readily become tolerant of low blood-glucose levels, reliance on urine tests and symptoms of hypoglycaemia as a guide to dosage easily produces a spiral of overtreatment.