The transitional mucosa adjacent to colorectal carcinoma shows characteristic morphological and histochemical abnormalities that may be indicative of premalignant changes. Sixteen colorectal carcinomas were studied to analyse the proliferative activity and its distribution pattern in tissue samples from transitional mucosa located within 2 cm of the cancer and from uninvolved mucosa 10 cm from the cancer. Proliferative activity was assessed using a monoclonal antibody to proliferating cell nuclear antigen (PCNA). The total labelling index (percentage of labelled cells in a column) of transitional mucosa was not significantly different from that of uninvolved mucosa and normal control mucosa but it was significantly higher in cancer tissue (P < 0.001). An upward shift of the compartment of proliferating epithelial cells towards the bowel lumen was seen in transitional mucosa. The labelling index of the middle compartment (compartment 3) of the crypt in transitional mucosa was significantly higher than that in uninvolved mucosa and control mucosa (P < 0.001). Unstable cytokinetic change may already have started in apparently normal transitional mucosa. Assessment of cell proliferation kinetics by PCNA immunostaining may be useful in screening for heightened risk of the development of colorectal cancer.