This study was designed to investigate whether the presence of non-insulin-dependent diabetes mellitus (NIDDM) or coronary heart disease (CHD) in probands have different effects on serum lipid, lipoprotein and apolipoprotein concentrations in the first-degree relatives. Altogether 161 probands (114 men, 47 women) and 788 first-degree relatives of these probands (174 brothers, 246 sisters, 180 sons, 188 daughters) were included in the analyses. The presence of NIDDM in the proband was associated with lowered total, LDL and HDL cholesterol and apolipoprotein A1 and elevated total triglyceride levels in the brothers (P less than 0.05) and elevated total and LDL cholesterol levels in the sisters (P less than 0.05). Total LDL and VLDL cholesterol and apolipoprotein B were higher (P less than 0.05) and HDL/total cholesterol ratio and apolipoprotein A1/B ratio lower (P less than 0.05) in the daughters of the nondiabetic and diabetic probands were pooled, CHD in the proband was associated particularly with low apolipoprotein A1/B ratio. In conclusion, (1) the presence of NIDDM in the proband appears to be associated in siblings with more profound lipid and lipoprotein changes (especially low HDL cholesterol and high total triglycerides) than a history of CHD in the proband, (2) a history of CHD in the proband is associated in children with apolipoprotein changes favouring atherosclerosis (low apolipoprotein A1, high apolipoprotein B, low apolipoprotein A1/B ratio). Different effects of a history of NIDDM and CHD in the proband on lipid, lipoprotein and apolipoprotein levels in the first-degree relatives warrants more population-based studies.