Aggressive fibromatosis (desmoid tumor) is a mesenchymal lesion originating from fascial, aponeurotic and muscular connective tissue. It rarely becomes histologically malignant. In this study we analyzed the cell cycle regulation proteins: pRb, p16, and proliferating antigens: Ki-67, PCNA, MCM5 with immunohistochemical method in archival material derived from 27 extra-abdominal (E-AD), 18 abdominal (AD) and 5 intra-abdominal (I-AD) cases of desmoid tumor. None of the examined cases (n=50) of aggressive fibromatosis was pRb-immunonegative. Heterogeneous expression of pRb was observed in 51.85% (14/27) of Group AD cases and in 5.56% (1/18) of Group E-AD cases; positive expression in 48,15% (13/27) of Group AD cases, in 94.44% (17/18) of Group E-AD cases, and in 100% (5/5) of Group I-AD cases. There were no negative cases for p16 staining in any of the examined groups. The number of heterogeneous cases in individual groups was: 33.33% (9/27) in Group AD, 50% (9/18) in Group E-AD and 40% (2/5) in Group I-AD, and positive cases: 66.67% (18/27), 50% (9/18) and 60% (3/5), respectively. Overexpression of PCNA was noted in 98% (49/50) of cases. The positive staining for Ki-67 protein was noted in 25.93% (7/27) in Group AD, in 16.67% (3/18) in Group E-AD and in 60% (3/5) in Group I-AD. None of the examined cases was immunopositive for MCM5 protein. The noted levels of pRb and p16 expression in desmoid cells reflect their function in cell cycle regulation. Probably the unsettled cell cycle progression, especially in G1 phase, is not the cause of aggressive fibromatosis pathogenesis.