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Disrupted human gut microbiota have been associated with the development of certain disease states, including obesity and ulcerative colitis (UC). Faecal microbiota transplantation (FMT) from healthy donors is a promising avenue to shift the microbiome profile of the recipient towards that of the donor, potentially ameliorating related symptoms. Several recent meta-analyses have investigated the clinical and microbial determinants that influence the retention of transplanted donor microbial strains within the recipient gut microbiome following FMT (i.e. engraftment). However, the specific factors that affect donor strain engraftment in different disease states require further exploration. Here, we perform a strain engraftment analysis on data from two multi-donor FMT clinical trials: the Gut Bugs Trial for obesity and the FOCUS Trial for UC. Using donor strain matching, the donor-recipient pairings of the FOCUS Trial were first predicted in a blinded manner. The subsequent, unblinded, strain engraftment analysis of both datasets highlighted a differential effect of donor-recipient microbiome complementarity on engraftment across the two disease cohorts; greater engraftment efficiency was associated with increased donor-recipient microbial similarity in the FOCUS Trial, and decreased similarity in the Gut Bugs Trial, suggesting that the factors influencing engraftment may differ across disease cohorts.