AIMS: Uncertainties remain on the biological and prognostic significance and therapeutic implications of loss in body weight (W-LOSS) in chronic heart failure (HF) patients. We assessed whether W-LOSS added additional prognostic value to classical clinical risk factors in two separate and large cohorts of patients with chronic HF. The factors associated with W-LOSS were studied. METHODS AND RESULTS: W-LOSS and estimated plasma volume changes were measured serially in the GISSI-HF (n = 6820) and Val-HeFT trials (n = 4892). In both studies, experiencing at least one episode of ≥5% W-LOSS during the first year of follow-up was considered a sign of wasting. In GISSI-HF, self-reported unintentional W-LOSS ≥2 kg between two consecutive clinical visits within 1 year was also considered a sign of wasting. W-LOSS occurred in 16.4% and 15.7% of the patients enrolled in GISSI-HF and Val-HeFT, respectively (unintentional ≥2 kg W-LOSS occurred in 18.9% in GISSI-HF). In multivariable analyses adjusting for a number of baseline covariates as well as for plasma volume changes, W-LOSS was found to be independently associated with mortality and adverse cardiovascular and non-cardiovascular outcomes, with a significant net reclassification improvement (cfNRI) and an increase in integrated discrimination improvement (IDI). W-LOSS was independently associated with several features representing the severity of HF, including baseline NT-proBNP and high sensitivity C-reactive protein (hsCRP) in Val-HeFT. CONCLUSIONS: W-LOSS was a frequent finding in the GISSI-HF and Val-HeFT trials, associated with multiple patient features, and added additional prognostic information beyond clinical variables of HF severity, including estimated plasma volume changes.