Onapristone is a progesterone antagonist which inhibits the growth of mammary tumours in mice. The effect of Onapristone and concomitant oestrogen (E 2 ) supplements on the growth of five human gastrointestinal cancer xenografts was examined. E 2 stimulated RD19 (gastric tumour) tumour growth in female mice and tumours grew less well when also treated with Onapristone (P < 0.05). Onapristone had no effect on male mice bearing RD19 tumours or mice of either sex bearing MKN45G (gastric) tumours. PAN-1 (pancreatic) tumours were significantly stimulated by E 2 (by 64% of control, P = 0.02) and Onapristone treatment inhibited E 2 stimulated growth (52% reduction of E 2 control, P > 0.05). C146 (colorectal) tumour growth was not stimulated by E 2 nor inhibited by Onapristone. E 2 stimulated formation of AP5LV (colorectal) tumour nodules (in lungs) (mean 38-52, P = 0.001). Onapristone significantly reduced the number of nodules (mean 32, P < 0.05) only in female mice not given E 2 . Xenografts of some GI tumour cell lines grow at different rates in male and female mice. E 2 may cause additional growth stimulation and E 2 stimulated growth can be reversed by Onapristone to basal levels.