The frequency of nocturnal hypoglycaemia, i.e. blood glucose concentration (BG) less than 3.0 mmol/l, was evaluated in consecutively selected insulin-dependent patients on multiple insulin injections (MII), n = 23, or continuous subcutaneous insulin infusions (CSII), n = 25. Blood was sampled hourly from 23.00 to 07.00. Seven patients (30%) on MII had at least one BG less than 3.0 mmol/l during the night. Eleven patients (44%) on CSII had hypoglycaemia (NS). The total number of BGs less than 3.0 mmol/l was higher on CSII, 42 of 225, versus 16 of 207 on MII (p less than 0.025). The duration of hypoglycaemia was 2 hours (range 1-6) on MII and 4 hours (range 1-7) on CSII with a maximal prevalence at 4 hours and between 5 and 7 hours, respectively (p = less than 0.05). The frequency of nocturnal hypoglycaemia is high in patients on intensified insulin regimens. Nocturnal hypoglycaemia occurs later in the night and is of longer duration on CSII than on MII. HbA1c, BG before bedtime and in the morning might be useful in the evaluation of nocturnal hypoglycaemia.