To examine whether weight loss at presentation influences outcome in patients who received chemotherapy for lung cancer or mesothelioma. Multivariate analysis of prospectively collected data 1994-2001. Data were available for age, gender, performance status, histology, stage, response, toxicity, progression-free and overall survival. The outcomes of patients with or without weight loss treated with chemotherapy for small cell lung cancer (SCLC; n=290), stages III and IV non-small-cell lung cancer (NSCLC; n=418), or mesothelioma (n=72) were compared. Weight loss was reported by 59, 58 and 76% of patients with SCLC, NSCLC and mesothelioma, respectively. Patients with weight loss and NSCLC (P=0.003) or mesothelioma (P=0.05) more frequently failed to complete at least three cycles of chemotherapy. Anaemia as a toxicity occurred significantly more frequently in NSCLC patients with weight loss (P=0.0003). The incidence of other toxicities was not significantly affected by weight loss. NSCLC patients with weight loss had fewer symptomatic responses (P=0.001). Mesothelioma patients with weight loss had fewer symptomatic (P=0.03) and objective responses (P=0.05). Weight loss was an independent predictor of shorter overall survival for patients with SCLC (P=0.003, relative risk (RR)=1.5), NSCLC (P=0.009, RR=1.33) and mesothelioma (P=0.03, RR=1.92) and an independent predictor of progression-free survival in patients with SCLC (P=0.01, RR=1.43). In conclusion, weight loss as a symptom of lung cancer predicts for toxicity from treatment and shorter survival.