BACKGROUND We studied the joint effect of baseline triglyceride and lipoprotein cholesterol levels on the incidence of cardiac end points in the trial group (n = 4,081) of the Helsinki Heart Study, a 5-year randomized coronary primary prevention trial among dyslipidemic middle-aged men. The relative risks (RR) were calculated using Cox proportional hazards models with a dummy variable technique that allows simultaneous study of subgroup combinations from the placebo and treatment groups. RESULTS In the placebo group (n = 2,045), the low density lipoprotein cholesterol (LDL-C)/high density lipoprotein cholesterol (HDL-C) ratio was the best single predictor of cardiac events. This ratio in combination with the serum triglyceride level revealed a high-risk subgroup: subjects with LDL-C/HDL-C ratio greater than 5 and triglycerides greater than 2.3 mmol/l had a RR of 3.8 (95% CI, 2.2-6.6) compared with those with LDL-C/HDL-C ratio less than or equal to 5 and triglyceride concentration less than or equal to 2.3 mmol/l. In subjects with triglyceride concentration greater than 2.3 mmol/l and LDL-C/HDL-C ratio less than or equal to 5, RR was close to unity (1.1), whereas in those with triglyceride level less than or equal to 2.3 mmol/l and LDL-C/HDL-C ratio greater than 5, RR was 1.2. The high-risk group with LDL-C/HDL-C ratio greater than 5 and triglyceride level greater than 2.3 mmol/l profited most from treatment with gemfibrozil, with a 71% lower incidence of coronary heart disease events than the corresponding placebo subgroup. In all other subgroups, the reduction in CHD incidence was substantially smaller. CONCLUSIONS Serum triglyceride concentration has prognostic value, both for assessing coronary heart disease risk and in predicting the effect of gemfibrozil treatment, especially when used in combination with HDL-C and LDL-C.