There are considerable similarities between atypical small gland hyperplasia and well-differentiated small-acinar carcinoma of the prostate. The distinction between these two entities must be made on the basis of both architectural and cytologic criteria. Alcian blue positive intraluminal acid mucin is a helpful diagnostic adjunct when it is present in small glandular proliferations together with intraluminal crystalloids. Their presence has provided reassurance in the diagnosis of carcinoma, particularly in cases with severe crush artifact and in cases of extremely well-differentiated adenocarcinoma. However, we emphasize that the finding of intraluminal acid mucin and crystalloids should be used only in conjunction with and not as a substitute for architectural and cytologic criteria in the differential diagnosis of small-acinar prostatic carcinoma.