The histological diagnosis of gastric leiomyosarcoma and the grading of its malignancy are based on anaplasia, mitotic counts, cellularity, etc, but generally are subjective. To establish more objective criteria, a monoclonal antibody to proliferating cell nuclear antigen (PCNA), PC10, which is a marker for proliferative activity of tumors, was applied to 21 myogenic tumors of the stomach (14 leiomyomas and 7 leiomyosarcomas) fixed in formalin. The reaction-time (1-4 minutes) for the staining was decided section by section, under direct observation of its inner controls. The PCNA labeling index of the proliferative zone of the normal fundic gland in each tumor-containing section showed no significant difference among three groups with different formalin fixation times (5 cases fixed 3 days or less, 10 cases fixed 4-10 days and 6 cases fixed 11 days or more), when their inner controls stained well. The PCNA labeling index of 7 leiomyosarcomas was higher than that of 14 leiomyomas (P < 0.01), and correlated well with the mitotic index (r = 0.98). An 8 mm cellular leiomyoma revealed a PCNA labeling index as high as that of low-grade leiomyosarcoma. It is concluded that PCNA staining is a useful indicator to distinguish leiomyosarcoma from leiomyoma and grade the malignancy of leiomyosarcoma, and that small leiomyosarcoma may be diagnosed as cellular leiomyoma in hematoxylin-eosin stained sections, because of low grade atypia in the early phase.