To investigate whether morning or evening injection of a long-acting insulin preparation (Ultratard HM, Novo) affects the glycaemic control in insulin-dependent diabetic (IDDM) patients, 24-hour blood glucose and plasma free insulin profiles were obtained in nine C-peptide negative IDDMs after one daily injection of insulin Ultratard HM either before breakfast (0800 h) or at 2200 h during the preceding 14 days. There was a tendency towards higher plasma free insulin levels and lower blood glucose values from 0100 h to 1300 h when Ultratard HM insulin was injected at 2200 h as compared to the morning injection. There was, however, only a significant difference between the corresponding plasma free insulin values at 1200 h, and blood glucose levels did not differ significantly at any time point investigated. The number of blood glucose values below 3 mmol/l in the study periods was 6 when insulin Ultratrad HM was injected at 2200 h as opposed to 2 when insulin was given at 0800 h, but this difference was not significant.