The original pre-treatment histological sections from 125 patients with invasive (T2/T3) transitional cell bladder cancer treated by radical radiotherapy were studied; 63 tumours responded completely to radiation and 62 did not; 55 of 72 tumours containing areas of squamous metaplasia and 27 of 36 staining for beta-human chorionic gonadotrophin failed to respond to radiotherapy; 26 of 28 tumours showing both squamous metaplasia and beta-human chorionic gonadotrophin did not respond to radiation, whereas 40 of 45 tumours without either of these features responded. The DNA ploidy of 86 tumours in the series was measured by flow cytometry; 11 of 27 aneuploid and 30 of 59 diploid tumours responded to irradiation. Squamous metaplasia and beta-human chorionic gonadotrophin in bladder cancer indicate resistance to radiotherapy but DNA ploidy does not.