The prognostic significance of pre-treatment serum concentration of squamous cell carcinoma antigen (SCCAg) was investigated in a population of 60 patients with anal epidermoid cancer. Serum concentration was determined during the period 1985-90. The patients were followed until December 1991. An antigen level above 2.0 ng/ml was classified as elevated. Thirty-eight patients had T1-2 tumours, while 22 had T3-4 tumours. Sixteen of the 20 patients (80%) with an elevated SCCAg concentration relapsed or had residual tumour after conclusion of primary therapy, compared to 8 of 40 (20%) with a normal concentration (P = 0.00001). Six patients of 40 (15%) with a normal SCCAg had died at the end of follow-up compared to 10 of 20 (50%) with an elevated value (P = 0.006). For patients with T1-2 tumours 2 of 29 (7%) with a normal SCCAg had died with or from their cancer compared to 3 of 9 (33%) with an elevated value (P = 0.14); for T3-4 tumours these figures were 3 of 11 (P = 0.14); for T3-4 tumours these figures were 3 of 11 (27%) and 7 of 11 (64%) respectively (P = 0.009). Actuarial survival analyses for patients with normal vs elevated SCCAg concentration showed that the projected five year overall survival was 81% for those with a normal level and 43% for those with an elevated level (P = 0.004). For tumour specific survival the figures were 83% vs 45% (P = 0.004).(ABSTRACT TRUNCATED AT 250 WORDS)