Menstrual cycles of women with endometriosis more frequently demonstrate two LH surges compared with the typical single surge recorded in control subjects (90% of endometriosis cycles showed biphasic LH surges in Cheesman et al., 1982, and 17% in Vaughan Williams et al., 1986), which contrasts with the absence of an LH surge, and corresponding anovulation, found in PCOS. This evidence concerns the gene PLOD1 and endometriosis.