Purpose. A case report of Legionella pneumophlla pneumonia associated with off-label use of ustekinumab in a patient with Crohn's disease (CD) is presented. Summary. A 57-year-old man with longstanding CD was hospitalized with a four-day history of fever (38.5 degrees C), dyspnea, left, pleuritic pain, and weight loss (more than 6 kg) about six weeks after beginning treatment with ustekinumab, a human monoclonal antibody approved in the United States for two indications (plaque psoriasis and psoriatic arthritis) and currently under investigation as a potential treatment for CD and other inflammatory disorders. During the preceding 25 years, the man had been treated for severe CD with a number of agents (e.g., infliximab, adalimumab, certolizumab); ultimately, off-label ustekinumab therapy (90 mg subcutaneously weekly) was initiated due to persistent severe CD symptoms. Chest x-ray studies at the time of admission demonstrated left upper lobar consolidation, and a urine antigen test was positive for L. pneumophlla. The patient was treated with i.v. levofloxacin and methylprednisolone and discharged after two weeks. Ustekinumab was reintroduced (45 mg subcutaneously every two weeks), and the patient continued to receive the drug for 16 months, with clinical remission of CD symptoms and no further adverse events. A literature search identified two case reports of pneumonia associated with ustekinumab use, but neither case involved L. pneumophila. Conclusion. Pneumonia caused by L. pneumophila developed in a patient with CD treated with ustekinumab. Pneumonia symptoms resolved after ustekinumab was discontinued.