We have studied and compared 316 mucinous and 45 signet ring cell carcinomas of the rectum with 413 non-mucinous carcinomas. Mucinous carcinomas were subdivided according to the amount of mucus which was gauged subjectively as either more or less than 75% of the tumour volume. Five year survivals for non-mucinous, mucinous (less than 75%), mucinous (greater than 75%) and signet ring cell carcinoma were 62%, 60%, 53% and 13%. Mucinous carcinomas (less than 75%) were relatively well differentiated and showed an age distribution identical to their non-mucinous counterparts, but differed in their strong association with villous adenoma. Mucinous carcinomas (greater than 75%) were less well differentiated and, like signet ring cell carcinomas, occurred in younger patients and showed no special association with villous adenoma. Clinically important and independent predictive variables were found by the method of multivariate regression analysis to be number of lymph node metastases, extent of spread in continuity, character of invasive margin and peritumoural lymphocytic infiltration. After adjustment for these factors, typing of rectal cancer as mucinous, non-mucinous and signet ring cell gave no additional, clinically useful prognostic information.