The effects of time of sampling on plasma lipids and lipoprotein cholesterol concentrations were investigated in 88 patients undergoing elective coronary angiography. Patients with a myocardial infarction or major surgery within 6 weeks before catheterization were excluded. All subjects were sampled in the fasting state at the time of arteriotomy before systemic heparinization and at least 30 days after discharge from the hospital (mean 275 days) in the free living state. No statistically significant differences were noted in total cholesterol (220 +/- 51 vs 226 +/- 48 mg/dl), triglycerides (191 +/- 77 vs 191 +/- 113 mg/dl) and calculated low-density lipoprotein cholesterol levels (149 +/- 46 vs 150 +/- 43 mg/dl). High-density lipoprotein cholesterol values were significantly lower (p less than 0.0001) in subjects sampled before catheterization than in the free living state (32 +/- 10 vs 37 +/- 10 mg/dl, mean change 14%). Moreover, the frequency of high-density lipoprotein cholesterol less than 35 mg/dl was 77% before catheterization and 44% in the free living state. This effect was neither due to beta-adrenergic drugs nor to the length of time between samplings. In view of these findings, a screening lipid profile for patients with coronary artery disease should be performed in the free living state.