Malnutrition resulting from chronic congestive heart failure (cardiac cachexia, CC) is not uncommon and contributes to mortality and morbidity especially of elderly people. The aetiology of cardiac cachexia is probably multifactorial. We have assessed whether malabsorption of fat is associated with CC and if so whether it is due to small-bowel bacterial overgrowth. Three groups of subjects were studied: 29 (20 women) patients (mean age 76.1 years) with controlled congestive heart failure and weight loss (CC); 14 (seven women) patients (mean age 74.0 years) with controlled congestive heart failure and no weight loss (non-cachexia, NON-CC); and 29 (20 women) healthy controls (mean age 74.9 years). Fast absorption was quantified using the cumulative 6 h 14CO2 exhalation in the 14C-triolein breath test and small-bowel bacterial overgrowth was quantified using the cumulative 8 h 14CO2 exhalation in the 14C-glycocholic acid breath test. The cumulative 6 h 14CO2 exhalation in the triolein breath test was reduced in the CC group (p = 0.001) implying impaired fat absorption. There was no evidence of small-bowel bacterial overgrowth in any group. Impaired absorption of fat was related to the clinical severity of heart failure and its duration. Impaired fat absorption is associated with cardiac cachexia. It is not due to small-bowel bacterial overgrowth. The aetiology of fat malabsorption in heart failure requires further studies.