Method: Immunohistochemical study of 46 cases of GISTs of different malignant potential, statistical analysis. Results: The count of PHH3-immunoreactive cells demonstrates the strong correlation (Gamma 0,756; p<0,05) with malignant potential of GISTs detected by standard parameters. It clearly separates true mitotic cells from apoptotic and piknotic nuclei. In all cases PHH3 count was slightly higher than the mitotic index in H&E stained slides. We have found that PPH3 demonstrated the two types of staining: nucleosomal dot-like type and mitotic homogenous type. The first type, probably, reflects the fraction of cells just before the prophase, which cannot be identified in H&E slides. Conclusion: Immunostaining of PPH3 is a useful additional marker in detection of proliferative activity in GISTs, helping to identify true dividing cells correctly.