Mucin production by prostatic carcinomas is not unusual and the identification of acidic mucin with an appropriate stain may be a useful adjunct in routine diagnostic surgical pathology. In our series of 53 cases of prostatic carcinoma, 20 (38%) showed significant acidic mucin production. This may allow differentiation from benign, hyperplastic tissues which produce neutral mucins only. A case of colloid carcinoma of prostate is described which satisfies Elbadawi's criteria for this uncommon variant. Some suggestions are made which, if rigorously applied, will permit the recognition of genuine cases of colloid carcinoma and of the prognostic significance of mucin‐rich tumour