OBJECTIVE Faecal microbiota transplant [FMT] is effective in treating recurrent Clostridioides difficile infection [CDI] and restores gut microbiota composition. This is unlikely to account for its entire mechanism of efficacy, as studies have shown that factors such as bile acids influence the risk of infection by affecting Clostridioides difficile germination. We therefore aimed to investigate longitudinal changes in the gut bile acid composition after FMT performed for recurrent CDI, in children with and without inflammatory bowel disease [IBD]. METHODS Eight children received FMT; five had underlying IBD. Primary and secondary faecal bile acids were measured by liquid chromatography-mass spectrometry in recipients [pre-FMT and longitudinally post-FMT for up to 6 months] and donors. RESULTS Pre-FMT, recipients had higher primary and lower secondary bile acid proportions compared with donors. Post-FMT, there was a gradual increase of secondary and decrease of primary bile acids. Whereas gut bacterial diversity had been shown to be restored in all children shortly after FMT, normalisation of bile acids to donor levels occurred only by 6 months. In children with IBD, although microbiota diversity returned to pre-FMT levels within 6 months, secondary bile acids remained at donor levels. CONCLUSIONS The differences in bile acid profiles compared with gut bacterial diversity post-FMT suggests that interactions between the two may be more complex than previously appreciated and may contribute to FMT efficacy in different ways. This initial finding demonstrates the need to further investigate gut metabolites in larger cohorts, with longitudinal sampling to understand the mechanisms of FMT effectiveness.