The serum levels of CA125 were highest in patients with ovarian cancer (mean 1117.1 U ml−1, P<0.001) but were also significantly (P<0.001) elevated in patients with ovarian endometrioma (44.3 U ml−1) and advanced non-ovarian endometriosis (ASRM stage 4, 40.8 U ml−1) as compared with healthy controls (8.9 U ml−1). The gene discussed is MUC16; the disease is ovarian endometriosis.