Carbohydrate antigen 19-9 (CA 19-9) is a glycoprotein of molecular weight 360,000-390,000 daltons which is closely related to Lewis blood antigen. It is normally secreted by the urothelium and supposed to have a protective function against the harmful effect of low pH on the cells. With the development of malignancy its secretion increases, secondary to anaerobic glycolysis and lactate accumulation, (differentiation trait). It is secreted by the pancreas and may show increase in some gastrointestinal malignancies. Carcinoma of the urinary bladder is a major problem in Egypt. It represents 26% of malignant tumours, 86% of tumours occur on top of bilharzial cystitis. It has three histopathological types: squamous cell carcinoma (75.9%), transitional cell carcinoma (TCC) (16%), adenocarcinoma (6%) and undifferentiated (2.1%). CA 19-9 serum level was studied in the different groups. It was found to be elevated only in TCC. The level was proportional to the bulk of malignant tissue and it returned to normal after tumour resection. Recurrence was associated with high serum level. CA 19-9 could be of value in detection and follow-up of TCC of the bladder.