Comparative morphological and cytogenetical investigations of eight human ovarian papillary adenocarcinomas were performed. By correlating various cytogenetical peculiarities of these tumors with their morphological patterns, the existence of three distinct tumor categories was noted. It was inferred that they represent three different morphocytogenetical stages of tumor evolution with the following characters: In the first stage papilliform excrescences prevail, covered by a single layer of atypical epithelium; the tumor cytogenetical structure is nearly diploid, and the cytogenetical alterations are scarce. In the second stage an intensely multilayered proliferation of malignant epithelium occurs, covering the papillary processes. Cytogenetically, besides nearly diploid metaphases, there are also heteroploid ones. In the third stage the papillary arrangement is lost in most tumor areas, and the malignant epithelium invades the stroma diffusely. A great number of giant cells show the important alterations occurring in this stage. The cytogenetical structure is heteroploid with different marker chromosomes. The necessity of determining the morphocytogenetical stage of tumor evolution is discussed.