In order to evaluate whether Lp(a), a lipoprotein that is potentially thrombogenic and atherogenic, is a potential risk factor for CAD in non-insulin-dependent diabetes (NIDDM), we compared the Lp(a) and its distribution in 145 NIDDM patients with that in 94 healthy control subjects. Furthermore, we studied the effect of insulin treatment on serum Lp(a) in 108 patients with NIDDM. Male and female NIDDM patients had similar Lp(a) concentrations to healthy controls (median value 167 mg L-1, range 15-1550 mg L-1 vs. 157 mg L-1, range 15-919 mg L-1, NS and 92, range 15-1190 mg L-1 vs. 103 mg L-1, range 15-842 mg L-1, NS). Also, the cumulative distribution of Lp(a) did not differ between the NIDDM patients and healthy subjects. Insulin treatment increased Lp(a) in diabetics with a Lp(a) concentration of less than 300 mg L-1, but this effect was not related to the concomitant improvement in metabolic control (mean change (+/- SEM) of HbA1c from 9.80 +/- 0.15 to 8.00 +/- 0.12; P < 0.001). In subjects with elevated Lp(a) concentrations (> 300 mg L-1) the Lp(a) concentration was unaffected by insulin, despite a similar improvement in glycaemic control. These results suggest that insulin may modulate the concentration of Lp(a).