We investigated 24 patients with melanotic tumours (under them 22 malignant melanoms). The tumour material (31 x by biopsy, 2 x by autopsy) was homogenized and stained with Ethidium bromide after pepsination. The distribution of the DNA content of the tumour cell nuclei was measured by impulse cytophotometry. The criterion for the curves was the height of the 4-c-peak in percentage of the diploid peak. Our material was divided in 3 groups: I. Without metastases. II. Metastases, slowly progressive. III. Metastases, rapidly progressive. Between the groups there were significant differences: the higher the 4-c-peak, the greater the malignancy. In the group III we mostly had tetraploid populations with the maximal peak at 4-c. The course of the disease (as seen by metastasis, effect of chemotherapy, growing in cell culture) runs parallel to the findings of impulse cytophotometry. In one case of melanosis praeblastomatosa circumscripta Dubreuilh we found a pure diploid cell population without peaks at 4-c. For prognosis (not for diagnosis) the impulse cytophotometric investigation of malignant melanoma is more suitable than the histology. In the discussion we expose the connections between proliferation and DNS distribution of cell nuclei in a tetraploid population. The terminology of cell cycle phases is extended to tetraploid populations with new terms (G', S'). In some cases we found stem lines. As shown by repeated impulse cytophotometric investigations a changing of stem lines in the DNA distribution curve is possible (change from hyperdiploid to tetraploid). The impulse cytophotometry is suitable for such investigations on melanomas.