Aggressive fibromatosis (desmoid tumor) is an uncommon locally invasive non-metastasizing neoplasm lesion. Desmoid tumor consists of fibroblasts, miofibroblasts and a significant amount of extracellular matrix. p27KIP1 (p27) protein is a member of the universal cyclin-dependent kinase inhibitor (CDKI) family that regulates progression through the cell cycle. In various human neoplasms the decreased level of p27 was observed. There were analysed 42 specimens of aggressive fibromatosis, in which there were 24 abdominal and 18 extra-abdominal cases. There was performed immunohistochemical analysis employing a monoclonal antibody against p27 protein and Ki-67 (Novocastra, UK). The sections for immunohistochemical study were stained using the streptavidin - biotin method. The average percentage of cells stained positively for all cases for p27 and Ki-67 was 22.1% (SD=29.2) and 6.0% (SD=8.8) respectively. There was no statistically significant difference between Ki-67 or p27 expression in abdominal and extra-abdominal location. Analysis of p27 and Ki-67 expression levels might indicate that low proliferating activity of desmoid fibroblasts is connected with another mechanism than the one, in which p27 takes part.