OUTLINE: This is a dose escalation and dose efficacy, multicenter study. In the dose efficacy portion, patients are stratified according to disease type (gastrointestinal stromal tumor vs all other soft tissue sarcomas). Phase I: Patients receive oral STI571 daily for a maximum of 24 months in the absence of disease progression or unacceptable toxicity. Cohorts of 3-8 patients receive escalating doses of STI571 until the maximum tolerated dose (MTD) is determined. The MTD is defined as the dose at which 2 of 6-8 patients experience dose limiting toxicities. The recommended phase II dose is defined as the dose preceding the MTD. Phase II: Patients receive the recommended phase II dose of STI571 as in phase I. Patients are followed every 8 weeks until disease progression, and then every 16 weeks thereafter.