Familial combined hyperlipidemia (FCHL) is a common lipid disorder characterized by an increase in cholesterol and/or triglyceride levels in multiple individuals of the same family. Prior reports document a decreased activity of lipoprotein lipase (LPL) in FCHL, and studies of the role of LPL in the remodeling of nascent lipoproteins suggest that disturbances in LPL function could underlie FCHL. We studied the LPL gene in 31 unrelated individuals with FCHL. A total of 25 DNA changes (13 "silent" substitutions and 12 DNA changes resulting in amino acid substitutions) were detected in 16 patients. Three new exonic polymorphisms as well as a previously described Ser447-->stop and an Asp9-->Asn substitution were seen with similar frequency on control and FCHL chromosomes. Two novel DNA changes resulting in an Asp21-->Val and an His44-->Tyr substitution were seen in only two FCHL individuals. In vitro studies showed no effect of these mutations on LPL catalytic activity. LPL mutations impairing catalytic activity did not represent a significant factor leading to FCHL in this population. Variations in any portion of the coding region of the LPL gene affecting other functions besides catalysis are not a frequent cause of FCHL.