The Southwest Oncology Group, in cooperation with the National Cancer Institute and the other major cooperative oncology groups, has participated in the development of new toxicity criteria for reporting the results of cancer clinical trials. The new criteria (NCI Common Toxicity Criteria) respond to a recognized need to: (1) report toxicities consistently among cooperative groups, particularly for shared 'intergroup' clinical trials; (2) strive for comparability in toxicity reporting among clinical trials; and (3) recognize new toxicities accompanying new classes of anticancer treatment modalities. The Southwest Oncology Group has extended the toxicity criteria, incorporating additional new criteria for biological agents, radiation therapy, surgery, and hormonal agents. In addition, endpoint definitions and response criteria are discussed. These have been developed in response to previous uncertainties in clinical trials objectives, to limitations in the resolution of imaging methods, and to demands for greater rigor in response and endpoint definitions. Toxicity criteria, endpoint definitions, and response criteria are tabulated.