The frequency of spontaneously occurring mitotic figures, the proportion atypical mitoses and spatial position (vertical, oblique or horizontal) of metaphasial plates were studied in 62 specimens: 47 with colorectal adenomas, and 15 with chronic ulcerative colitis (control cases). The characteristics of the atypical mitoses at pro-phase, pro-metaphase, metaphase, anaphase and telophase were defined and illustrated. The results indicated that atypical mitoses occurred in 8.9% in control patients, in 37.2% of the specimens tubular adenomas having low-grade dysplasia, in 80.8% in tubular adenomas with high-grade dysplasia, in 68.5% of villous adenomas with low-grade dysplasia and in 65.5% of villous adenomas with high-grade dysplasia. For villous adenomas with high-grade dysplasia having invasive growth, 81.3% of atypical mitoses was recorded. Oblique or horizontal metaphasial plates were present in 8% of the metaphases in control cases but in as much as 25% in tubular adenomas with low-grade dysplasia, in 37% of those with high-grade dysplasia, in 42% for villous adenomas with low-grade dysplasia, in 56% for those with high-grade dysplasia and in 53% for villous adenomas with invasive growth. The classification of the various degrees of dysplasia is at present done by the characteristic of the dysplastic cells at interphase. This paper demonstrates that qualitative and quantitative morphological alterations of the mitotic apparatus takes place in the various histological types of colorectal adenomas. Perhaps both the number of atypical mitoses and the spatial position of the mitoses should be registered in attempts to shed more light on the biological behaviour of adenomatous lesions of the colorectal mucosa.