Chemotherapy exerts a variable effect on nutritional status. It is not known whether loss of body fat or fat-free mass (FFM) during chemotherapy relates to diminished dietary intake, failure to meet elevated energy requirements, or to the presence of an acute-phase response. We sought to determine prospective measurements of body mass and composition, resting energy expenditure, energy and protein intake, and C-reactive protein over a course of chemotherapy in 82 patients with advanced cancer. There was a large dropout from the study. Prospective measurements were obtained in 19 patients with non-small-cell lung cancer (NSCLC), 12 with metastatic melanoma and 10 with metastatic breast cancer. There were significant increases in energy intake among patients with metastatic breast cancer, 873 (266-1480) kJ (mean 95% CI; P<0.01), and metastatic melanoma, 2513 (523-4503) kJ (P<0.01). Breast cancer patients gained percentage body fat over the course of treatment, 2.1 (0.8-3.5%). Gain or loss of body fat correlated to mean energy intake throughout chemotherapy in patients with NSCLC (Rs=0.751; P<0.01) and metastatic breast cancer (Rs=0.617; P<0.05). The ability to meet or exceed energy requirements led to gains in body fat among patients with metastatic breast cancer and NSCLC, but did not prevent loss of FFM in these groups.