Inflammatory myofibroblastic tumors (IMTs) are uncommon spindle cell proliferations that occur in the viscera and soft tissue of children and young adults. Their biologic potential is indeterminate: 25% of IMTs recur, and rare examples undergo malignant transformation (MT). This study investigates histologic features, DNA ploidy, and expression of apoptotic regulatory and oncogenic proteins in IMTs in an attempt to identify those deviances that might correlate with aggressive behavior or MT. Twenty-four formalin-fixed, paraffin-embedded IMTs for which clinical outcome was known were evaluated for cellularity, cytologic atypia, mitoses, ganglion-like cells, inflammatory infiltrate, DNA ploidy by flow cytometric examination, and bax, bcl-2, p53, and c-myc expression by immunohistochemical analysis. Sixteen (67%) of the IMTs did not recur, 6 (25%) recurred, and 2 (8%) underwent MT. Cellular atypia was observed in 69% of the cases without recurrence, 100% with recurrence, and 100% with MT. Ganglion-like cells were present in 56, 100, and 100% of the IMTs without recurrence, with recurrence, and with MT, respectively. There was no difference in the degree of cellularity, mitoses, or inflammatory infiltrate among the outcome groups. All of the tumors expressed bax, and none expressed c-myc. Two (8%) were immunoreactive for p53, one of which recurred and one of which underwent MT. bcl-2 expression was observed in 9 (37%) of the IMTs, with no difference among the three groups. Two IMTs were aneuploid, one of which underwent MT. Neither morphologic evaluation for cellularity, mitosis, or inflammatory infiltrate nor expression of bax or c-myc were useful for prediction of clinical outcome. The combination of atypia, ganglion-like cells, p53 expression and DNA ploidy analysis, however, might be useful to identify IMTs that might undergo MT or pursue a more aggressive clinical course with recurrences.