The prognostic effect of weight loss prior to chemotherapy was analyzed using data from 3,047 patients enrolled in 12 chemotherapy protocols of the Eastern Cooperative Oncology Group. The frequency of weight loss ranged from 31 percent for favorable non-Hodgkin's lymphoma to 87 percent in gastric cancer. Median survival was significantly shorter in nine protocols for the patients with weight loss compared to the patients with no weight loss. Chemotherapy response rates were lower in the patients with weight loss, but only in patients with breast cancer was this difference significant. Decreasing weight was correlated with decreasing performance status except for patients with pancreatic and gastric cancer. Within performance status categories, weight loss was associated with decreased median survival. The frequency of weight loss increased with increasing number of anatomic sites involved with metastases, but within categories of anatomic involvement, weight loss was associated with decreased median survival. These observations emphasize the prognostic effect of weight loss, especially in patients with a favorable performance status or a limited anatomic involvement with tumor.