There are many arguments to support the hypothesis that there is a causal relationship between the presence of endometriosis and subfertility. These arguments are reviewed in this article and include: (1) an increased prevalence of endometriosis in subfertile women compared with women of proven fertility; (2) a reduced monthly fecundity rate (MFR) in baboons with mild to severe (spontaneous or induced) endometriosis compared with those with minimal endometriosis or a normal pelvis; (3) a trend toward a reduced MFR in infertile women with minimal to mild endometriosis compared with women with unexplained infertility; (4) a dose-effect relationship: a negative correlation between the r-AFS stage of endometriosis and the monthly fecundity rate and crude pregnancy rate; (5) a reduced monthly fecundity rate and cumulative pregnancy rate after donor sperm insemination in women with minimal-mild endometriosis compared with those with a normal pelvis; (6) a reduced MFR after husband sperm insemination in women with minimal to mild endometriosis compared with those with a normal pelvis; (7) a reduced implantation rate per embryo after IVF in women with moderate to severe endometriosis compared with women with a normal pelvis; and (8) an increased monthly fecundity rate and cumulative pregnancy rate after surgical removal of minimal to mild endometriosis.