The records of 100 recent urology service patients with bladder neoplasms were evaluated for correlation of cytological findings with clinical and histological diagnoses. At the time of diagnosis 83 patients had synchronous positive biopsies and cytologies, 3 had negative cytologies (even though urothelial cancer was present) and the remaining 14 had abnormal cytologies but clinical examination was negative. Of these 14 patients invasive cancer has been identified subsequently in 11, while 3 have not yet returned for clinical followup. Analysis of the 11 patients in whom cancer was proved eventually reveals 14 distinct periods, ranging from 1 to 37 months, in which cytologies were abnormal and concurrent clinical examinations and histological diagnoses were negative. Ten of these periods preceded tumor recurrence and the other 4 antedated the first occurrence of cancer. A relationship was found between a cytological diagnosis of carcinoma in situ or severe dysplasia and the presence or future appearance of low grade clinical cancer. Cytologies containing overtly malignant cells more commonly were associated with or preceded the appearance of high grade cancers. In these 11 patients the sensitivity of a single spontaneously voided urine specimen was 97 per cent but the sensitivity was 100 per cent when 2 or more specimens were submitted per patient. Bladder barbotages were 100 per cent sensitive.