Flow-through fluorescence cytophotometric determination of nuclear DNA content was employed for the diagnosis of prostate carcinoma. Fine needle aspiration biopsy material from the prostate of 220 patients was used for study. A false negative rate of 11.4% and a false positive rate of 29.7% were obtained when the results of flow-through photometry were compared with those of traditional cytodiagnosis. It was found that 4.5% of the specimens were unsuitable for cytologic diagnosis and 10.9% for flow-through cytophotometry. False negative DNA histograms may be due to two factors: either the number of tumor cells is small or there are tumor cells whose nuclear DNA content does not differ from that of a normal cell population. False positive findings result from proliferating cells in inflammatory activation. Errors in preparation of the material and mechanical mistakes, such as cellular clumping and coincidences, are less likely causes. The greater percentage of specimens which were inadequate for cytophotometry was due to the large number of cells needed for a utilizable flow-through photometric histogram. The high rate of false negative and false positive results (11.4% and 29.7%, respectively) argues against using flow-through photometric nuclear DNA determination for the diagnosis of prostate carcinoma.