Objective To evaluate serum MUC1 levels (a high molecular weight glycoprotein which is upregulated and abnormally glycosylated in bladder cancer and other carcinomas) in patients with a variety of stages and grades of transitional cell carcinoma (TCC) of the bladder, to assess its potential as a tumour marker. Patients and methods Blood samples were taken before treatment in 87 patients with TCC of the bladder and in 31 controls undergoing cystoscopy for benign conditions. Serum MUC1 levels were estimated with an enzyme‐linked immunosorbent assay using the C595 monoclonal antibody. Results Of patients with T4 tumours, 47% had MUC1 levels above the normal range (P<0.001); patients with T3 tumours also had significantly higher MUC1 levels than controls. The overall sensitivity was only 24% for all tumours when the upper limit of normal was defined as 4.8 U/mL; the specificity was 97%. Conclusion Serum MUC1 is not as useful tumour marker for screening, as it has a low sensitivity. However, MUC1 levels are high in advanced disease and serum MUC1 levels may be useful for disease monitoring.