Plasma and lipoprotein cholesterol and triglycerides, glucose and haemoglobin A_1c concentrations were measured in 106 patients (56 males) with insulin-dependent diabetes mellitus (age range 2–22 years) and 36 normal volunteers (19 males) with similar age and sex distribution. The diabetic patients were further divided into three subgroups: good, fair and poor control, based on 24 h glycosuria and haemoglobin A_1c concentrations. Total, low density lipoprotein and very low density lipoprotein cholesterol levels were significantly increased in male patients in poor control when compared with the group in good control and with normal subjects. Triglyceride and very low density lipoprotein triglyceride levels were also significantly increased in poorly controlled males. The most significant difference however was a decrease of high density lipoprotein cholesterol in male patients in poor control. There was a significant inverse correlation between haemoglobin A_1c and high density lipoprotein cholesterol (r = -0.63) and a direct correlation between haemoglobin A_1c and low density lipoprotein cholesterol (r = 0.35) and triglycerides (r = 0.62) in the male diabetics. The findings were similar in females. The most striking change was observed in low density lipoprotein cholesterol levels, which were more markedly increased in poorly controlled females than in poorly controlled males. No statistical singificant differences were found between the groups in good and fair control for any of the plasma and lipoprotein lipids studied. A significant difference however was found between the groups in poor and fair control. There was a significant correlation in females between haemoglobin A_1c and low density lipoprotein cholesterol levels (r = 0.43), haemoglobin A_1c and triglycerides levels (r = 0.54) and an inverse correlation between haemoglobin A_1c and high density lipoprotein cholesterol (r = -0.59).