The DNA content and proliferative index of 61 gastrointestinal stromal tumors (GIST) were measured by image analysis and correlated with the lesion's clinicopathological features and patient's survival. DNA analysis was performed on cytospin single-cell preparations obtained from the paraffin-embedded tissue blocks. MIB-1 was the proliferation marker used on paraffin sections. DNA aneuploidy was detected in 12 tumors (18%), and high MIB-1 index (>22%) in 12 lesions (18%). DNA aneuploidy and high MIB-1 index statistically correlated with high mitotic rate (> or = 5 x 10 high-power field [HPF]) (P < .001) and with the presence of necrosis (P < .05). The patient's survival was significantly correlated with DNA ploidy (P < .01), MIB-1 index (P < .00001), mitotic rate (P < .00001), presence of necrosis (P < .0001), and size of the tumor (P < .01). Multivariate regression analysis showed that only MIB-1 index was an independent parameter in predicting the clinical outcome for patients with GIST. The mitotic rate was the only other independent prognostic factor when MIB-1 index was not allowed to enter the model.