The expression of ABH blood group isoantigen was determined with the avidin-biotin-peroxidase complex method in a retrospective consecutive material of 230 patients with transitional cell bladder carcinoma. The follow-up period was 5 to 9 years. The five-year corrected survival for 65 patients whose primary tumours showed predominant expression of ABH antigens ('positive') was 80% and for 146 patients with predominant deletion ('negative') 60% (p less than 0.01). Of 107 patients with superficial tumours there were 60 negative and 47 positive. In an analysis of early progression, the negative tumours were found to progress more frequently than positive ones (p less than 0.03). Twenty-three per cent of the negative tumours finally progressed during the whole follow-up, compared with 15% of the positive ones; this difference was not significant. Almost all progressing tumours became negative when they reached an advanced stage.