To investigate the left- and right-sided distribution of ovarian malignant surface epithelial tumours, data were collected on 209 women undergoing first-line surgery for Stage I and II disease. Considering the unilateral cancers, the observed proportion of left-sided lesions was 35/54 (65%) in the endometrioid, 20/45 (44%) in the serous, 19/35 (54%) in the clear cell, 13/29 (45%) in the mucinous, 2/8 (25%) in the mixed, and 2/5 (40%) in the undifferentiated histological type group. The proportion of left-sided unilateral endometrioid cancers was significantly different from the expected 50% (chi2(1), 4.74, P = 0.03) and very similar to that previously observed for benign endometriotic cysts, constituting further evidence in favour of a possible development of endometrioid cancers from the latter lesions.