Flow-through cytophotometric determination of nuclear DNA content on jet wash material from the endometrium was employed in the diagnosis of endometrial carcinoma. One hundred and thirty cases were studied. In comparison to results from cytodiagnosis, flow-through photometry yielded a false negative rate of 31.6% and a false positive rate of 42.2%. The false negative findings resulted in part from the small relative frequency of atypical cells in a mixed population of normal and atypical cells in certain cases. Besides this, we often found carcinomas with a diploid DNA stem line, which could not be distinguished cytophotometrically from normal corpus endometrium (Sandritter, 1952; Atkin et al., 1959; Hustin, 1976). The flow-through photometrically false positive findings may have resulted either from cell aggregates or from a nuclear DNA content elevated over the diploid value in proliferating cells (D. Wagner et al., 1968; D. Wagner and Richard, 1968). The observed false negative and false positive rates demonstrate that flow-through photometric determination of nuclear DNA content is unsuitable for the diagnosis of endometrial carcinoma.