Elevated serum lipoprotein(a) [Lp(a)] levels increase the risk of cardiovascular disease if levels of low-density lipoproteins (LDLs) are also high. The biological function of Lp(a) is unknown, but plasma levels may be elevated in inflammatory disease. Endometriosis is a common gynecologic disorder in which endometrial tissue is found outside of the lining of the uterine cavity. There is an immune component to this condition whereby the number of peritoneal macrophages is increased and the level of prostanoids and cytokines in peritoneal fluid is elevated. In the present study, we measured serum lipid, lipoprotein, and apolipoprotein levels in 29 women with endometriosis and in 29 matched healthy controls. Fasting serum triglyceride and apolipoprotein (apo) Al levels were higher in women with endometriosis (+28.1%, P < .001, and +12.3%, P < .01, respectively), but there were no significant differences in LDL or high-density lipoprotein (HDL) cholesterol levels. Serum Lp(a) levels were fivefold higher (P < .01) in the patients (median, 15.0 mg/dL; range, 0.05 to 60.0) than in controls (median, 3.1 mg/dL; range, 0.05 to 57.2). The distribution of apo(a) isoforms was similar in the two groups, but in women with endometriosis the individual apo(a) isoforms tended to be associated with higher serum Lp(a) levels. Endometriosis may represent a relatively common condition in which to investigate the role of Lp(a) in human metabolism.