Opportunistic infection is a major threat to immunocompromised patients. However, infection due to Mycobacterium xenopi is rare in renal transplant recipients. We report two new cases of M. xenopi pulmonary infection (one case of interstitial pneumopathy and one of a pulmonary nodule) in renal transplant recipients, detected in the same center at an interval of a few months. Both patients were on an immunosuppressive regimen including a recent switch to sirolimus. Sirolimus is a new immunosuppressive drug already known to be responsible for interstitial pneumonitis. It also inhibits interleukin-12-induced proliferation of activated T lymphocytes, which is critical for the development of the cell-mediated immunity that protects against mycobacteria. These two case reports suggest that sirolimus therapy may lead to an impairment of the immune response against intracellular pathogens such as M. xenopi.