SUMMARYFasting lipid concentrations have been measured in fifty treated juvenile diabetics, their siblings and parents to determine which types of hyperlipoproteinaemia co‐exist with juvenile diabetes and whether the abnormalities relate to diabetic control, or represent familial disorders. Lipid concentrations amongst the parents did not differ from adult controls. Triglyceride concentrations were significantly higher in those diabetic children with fasting blood glucose concentrations>10 mmol/l than those with concentrations<10 mmol/l. The latter group had similar trigly‐ceride levels to non‐diabetic siblings. Cholesterol concentrations were not related to fasting blood glucose and were similar in diabetic and sibling controls. Hyperlipoproteinaemia (types IIa, IIb and IV) was present in ten of the diabetic patients. Six of the nine diabetic patients with raised cholesterol had at least one parent with cholesterol in the highest quintile for the control population, whereas only six of the forty‐one with lower levels had parents in this category. A similar trend for cholesterol was apparent amongst the non‐diabetic siblings. However, no association was apparent between the triglyceride levels of diabetics (or their siblings) and parents. Thus although hyperlipidaemia associated with juvenile diabetes appears to be largely due to inadequate control, raised cholesterol concentrations frequen