PURPOSE: Anti-human leukocyte antigen (HLA) immunoglobulin (Ig) M production stimulated by an alloantigen is sensitive, making IgM a novel potential marker of allorejection after organ transplantation. This study examined the relationship between the serum levels of anti-HLA IgM early after clinical lung transplantation (LTx) and the post-transplant outcomes. METHODS: Thirty-one consecutive patients who underwent deceased LTx were included. Immunoreactivity against HLA was retrospectively analyzed by measuring the anti-HLA IgM levels in the serum sampled for the first 14 days after LTx. The flow panel reactive antibody technique was used. The ratio of the anti-class I IgM level at each day to baseline was obtained, and the peak IgM level was determined for each case. The correlation between the peak IgM level and subsequent development of acute rejection (AR), chronic lung allograft dysfunction (CLAD), and survival outcomes were examined. RESULTS: The peak IgM level was a significant risk factor for AR within 90 days in univariate and multivariate analyses. In the long term, the patients with positive IgM (peak level > 1.8) tended to have a poorer CLAD-free and overall survival than those with negative IgM. CONCLUSION: Elevation of anti-HLA IgM levels early after LTx may be correlated with a higher incidence of rejection and negative clinical outcomes.