Among 59 laparoscopically staged endometriosis patients, 28.8% tested positive for antinuclear antibody. Of 44 patients, 45.5% were lupus anticoagulant positive (greater than 1.3) and 20.5% were within a borderline range (1.2-1.3). Antinuclear antibody positivity was inversely related to stage of disease (P = .009); lupus anticoagulant positivity exhibited a similar trend, but did not reach statistical significance. Of 31 endometriosis patients, 64.5% exhibited immunoglobulin G (IgG) autoantibodies and 45.2% demonstrated IgM autoantibodies to at least one of 16 antigens investigated. Among IgG autoantibodies, those to phospholipids were most frequently detected, followed in order of frequency by antibodies to histones and nucleotides. The incidence of IgM autoantibodies was inverted, with antinucleotides appearing most frequently and antiphospholipids least frequently. A strong correlation was noted between the presence of lupus anticoagulant and antinuclear antibody with both IgG and IgM autoantibodies. These observations suggest that endometriosis is associated with abnormal polyclonal B cell activation, a classic characteristic of autoimmune disease. This contention is further supported in that immunoglobulin levels (particularly IgG) are elevated in patients with endometriosis, and more so in lupus anticoagulant-positive than lupus anticoagulant-negative endometriosis patients (P = .021).