Plasmacytoma (Pm) is a plasma cell (PC) neoplasia consisting of PCs and some of them show progression to multiple myeloma. But there is no clear indicators predicting this progression. In this study, MUC1 expression was evaluated in Pm cases to determine the predictive value and was compared with histopathologic grade which is known to be a prognostic indicator in Pm. Nine of 31 cases with Pm showed MUC1 expression. Only two of the 18 cases with mature morphology showed MUC1 expression while seven of 13 cases with immature morphology showed MUC1 expression and this was statistically significant (P < 0.006). Additionally, four of 11 cases with BM involvement showed MUC1 expression while five of 20 cases without BM involvement showed MUC1 expression. There was a trend MUC1 expression with BM involvement but there was not statistically significant association between MUC1 expression and BM involvement. We found that MUC1 expression is associated with immature morphology which is an important prognostic indicator in Pm and by analogy MUC1 expression may be an additional prognostic indicator in patients with Pm. (C) 2001 Elsevier Science Ltd. All rights reserved.