The [14C]-triolein breath test is used as a test of fat absorption. However, its validity has not been established. The aim of this study was to investigate, whether the absorption of [14C]-triolein could be estimated from the breath test, and whether the breath test could be useful as a clinical test. The [14C]-triolein absorption was estimated from faecal measurements, using 51CrCl3 as non-absorbable marker. The breath test was done according to the standard technique with hourly estimations of the 14CO2 expiration. Fifty-one patients participated. A nearly perpendicular, curvilinear relation between the 6-h cumulative 14CO2 expiration and the [14C]-triolein absorption was found, and no obvious cut-off level for normal 14CO2 expiration could be identified. Accordingly, the diagnostic sensitivity of the breath test was 80% at the expense of a specificity of 45%. In 19 patients duplicate measurements were done. A high intra- and inter-individual variation in the fraction of absorbed [14C]-triolein, expired within 6 h, was found. It is concluded that expiration of 14CO2 is influenced by factors other than the absorption of [14C]-triolein, and that the [14C]-triolein breath test is not useful as test of fat absorption.