Histological sections of 102 stromal neoplasms of the gastrointestinal tract occurring in 100 patients have been assessed for 23 clinical and histological parameters and the corresponding paraffin embedded (archival) material processed for flow cytometry. Where possible, information as to clinical presentation and survival was obtained. The only absolute criterion for malignancy was the presence of spread of tumour beyond the organ of origin at the time of diagnosis. Of the remaining tumours (i.e., tumours locally confined at diagnosis), those found incidentally at operation and those of a small size (less than 60 mm diameter) behaved in a generally benign fashion. Of the histological parameters, six correlated with malignant behaviour: high mitotic count, high cellularity, marked nuclear pleomorphism, rounded as opposed to spindle cell shape, bizarre mitoses, and vascular invasion. The presence of DNA aneuploidy as shown by flow cytometry correlated strongly with a poor prognosis (p less than 0.0005). Tumours with a high mitotic count [greater than 9 per 10 high-power fields (hpf) (1.59 mm2)] behaved in an almost uniformly malignant fashion. Those with a low mitotic count [less than 3/10hpf (1.59 mm2)], behaved in a benign fashion apart from one case where no mitoses were discernible yet the tumour metastasised and killed the patient. The intermediate group of tumours (3-9 mitoses per 10 hpf inclusive) were difficult to predict, although the majority behaved in a malignant fashion. Within this group the presence of DNA aneuploidy appeared most useful in predicting prognosis.