['Human bladder carcinoma is thought to arise from a field change that affects the entire urothelium. Whether independently transformed urothelial cell populations exist in the same patient is uncertain.', 'We studied the clonality of urinary bladder carcinoma in 18 female patients who underwent cystectomy for urothelial carcinoma. None had multiple tumors. Tumor samples were obtained from different areas of the same tumor. Sixty-seven tumor samples were analyzed. Tumor genomic DNA was microdissected and extracted from formalin-fixed, paraffin-embedded slides. The clonality of urothelial tumors was evaluated on the basis of a polymorphism of the X chromosome-linked human androgen receptor gene (HUMARA) locus. The technique is dependent on digestion of DNA with the methylation-sensitive restriction enzyme HhaI, polymerase chain reaction (PCR) amplification of HUMARA locus, and detection of methylation of this locus. With this method, only the methylated HUMARA allele is selectively amplified by PCR.', 'Eleven of 18 patients were informative. Nonrandom inactivation of the X chromosome was found in 9 of the 11 informative patients (82%). Seven patients showed different patterns of nonrandom X chromosome inactivation for tumor samples obtained from different regions of the same tumor. Two patients showed the same pattern of nonrandom X chromosome inactivation in all samples.'] Does precise microdissection of human bladder carcinomas reveal divergent tumor subclones in the same tumor?