Endometriosis and infertility are known to be associated, but it is unclear whether endometriosis causes infertility. We used contrast analogue enhancement to study the effect of peritoneal fluid from women with early stage endometriosis on the ciliary beat frequency of human fallopian tube epithelium. We obtained peritoneal fluid from six women with early stage endometriosis and from six fertile women with no evidence of endometriosis to use as controls. Fallopian tubes from hysterectomy specimens were collected from 17 women. The difference in ciliary beat frequency between fallopian tubes exposed to peritoneal fluids of women with and without endometriosis increased with the duration of incubation (mean difference at 24 h 1.35 Hz, 95% CI 0.94-1.75, p=0.01). At 24 h, ciliary beat frequency was significantly lower in the incubations with peritoneal fluid from women with endometriosis than controls (4.29 [0.15] vs 5.64 Hz [0.15], respectively, p=0.001). Impairment of ciliary action in women with endometriosis might reduce fertility.