AIMS: To compare levels of insulin antibodies in children and adolescents after initiation of insulin therapy using either insulin aspart (IAsp) or human insulin (HI) in combination with Neutral Protamine Hagedorn (NPH) insulin, and to investigate the relationships between insulin antibodies and HbA(1c) and insulin dose. METHODS: IAsp-specific antibodies (IAsp-Ab) and antibodies cross-reacting with HI and IAsp (HI-cross-Ab) were analysed by radioimmunoassay at diagnosis of diabetes and every 3-6 months for 30 months. Seventy-two patients (HI = 30, IAsp = 42) with Type 1 diabetes, aged 2-17 years were included. Data on HbA(1c), insulin dose and serious adverse events (SAEs) were collected retrospectively. RESULTS: IAsp-Ab levels remained low throughout the study. After 9 months, the level of HI-cross-Ab increased [mean (SD) HI, 48.8% (21.53), IAsp, 40.2% (17.92)] and remained elevated. Repeated measurement analysis of HI-cross-Ab levels showed no significant difference between treatments (P = 0.16). HI-cross-Ab were significantly associated with total insulin dose (U/kg) (P = 0.001) and time (P < 0.0001), but not with HbA(1c) (P = 0.24). Mean (+/- SD) HbA(1c) was similar at diagnosis (HI 9.5 +/- 1.97%, IAsp 9.6 +/- 1.62%), HbA(1c) then decreased and stabilized to about 6.0% in both groups. Few SAEs were reported, the majority being hypoglycaemic episodes. CONCLUSIONS: Treatment with IAsp and with HI was associated with an increase in HI-cross-Ab in insulin-naive children, but this did not influence treatment efficacy or safety. These results support the safe use of IAsp in children and adolescents with Type 1 diabetes.