HMGI(Y) proteins are overexpressed in experimental and human malignancies, including colon, prostate and thyroid carcinomas. To determine at which step of the carcinogenic process HMGI(Y) induction occurs, we analysed the expression of the HMGI(Y) proteins in hyperplastic, preneoplastic and neoplastic tissues of colorectal origin by immunohistochemistry. All the colorectal carcinomas were HMGI(Y)‐positive, whereas no expression was detected in normal colon mucosa tissue. HMGI(Y) expression in adenomas was closely correlated with the degree of cellular atypia. Only 2 of the 18 non‐neoplastic polyps tested were HMGI(Y)‐positive. These data indicate that HMGI(Y) protein induction is associated with the early stages of neoplastic transformation of colon cells and only rarely with colon cell hyperproliferation. © 2001 Wiley‐Liss, Inc.