Background and objective: Detailed body composition, such as fat-free mass, has not been examined in idiopathic pulmonary fibrosis (IPF). We investigated whether the fat-free mass index (FFMI), an index of lean body mass, predicted survival. Methods: Forty-four patients with IPF were enrolled in the study. Their body composition was assessed using direct segmental multi-frequency bioelectrical impedance analysis. The degree of correlation between variables of body composition and other variables such as forced vital capacity (FVC) and survival was examined. Results: There was a significant positive correlation between FFMI and FVC, diffusion capacity of the lung for carbon monoxide (DLCO) and 6-min walk distance, and a significant negative correlation with age. However, there was no significant correlation between FFMI and percentage predicted FVC or DLCO, with the degree of correlation being similar to that observed for BMI. The mean observation period in the survival analysis was 837.5 +/- 407.5 days. A univariate Cox proportional hazard model showed that several variables, but not BMI, were associated significantly with survival. FFMI (hazard ratio (HR): 0.64, 95% CI: 0.43-0.94, P = 0.02) and percentage predicted FVC (HR: 0.96, 95% CI: 0.93-0.99, P= 0.008) were significant factors in a multivariate model. Conclusion: We conclude that FFMI is a significant independent predictor of survival in patients with IPF.