A previous study reported that heterozygotes for lipoprotein lipase (LPL) deficiency have reduced LPL, the lipoprotein pattern classified as familial combined hyperlipidemia (FCHL), elevated apolipoprotein (apo) B levels, and reduced high density lipoprotein (HDL) levels. These findings suggest that subjects with reduced LPL may form one subset of the FCHL population. The purpose of the present study is to determine whether a subset of patients with FCHL have reduced LPL. Three patient populations with FCHL were studied: 1) subjects with the diagnosis of FCHL (n = 9) established by previous family studies, 2) clinic patients with a tentative diagnosis of FCHL (n = 14), and 3) subjects undergoing angiography who had coronary artery disease (CAD) and a diagnosis of FCHL by family study (n = 33). Two of nine subjects with the established diagnosis of FCHL, five of the 14 FCHL clinic patients, and 13 of the 33 CAD subjects with FCHL had reduced LPL activity in the same range as do individuals who are obligate heterozygotes for LPL deficiency. Subjects with FCHL and reduced LPL had higher plasma triglyceride (p < 0.01) and lower HDL cholesterol (p < 0.025) levels than did the subjects with FCHL and normal LPL levels (327 +/- 201 versus 210 +/- 122 mg/dl [mean +/- SD] and 36 +/- 7 versus 44 +/- 13 mg/dl, respectively). Thus, in all three groups of patients with apparent FCHL, 20 of 56 subjects (36%) had reduced LPL, suggesting that one subset of the FCHL population may be identified by an abnormality in LPL activity that is associated with lipoprotein abnormalities.