Among patients with cutaneous T-cell lymphoma (CTCL), sepsis and pulmonary infections are the first cause of death. We report on a patient with CTCL who, after more than 10 years of aggressive antineoplastic treatments, showed extensive pulmonary infiltrations on staging CT scan. Repeated CT scans were inconclusive for an infectious process, and the patient was still asymptomatic. The diagnosis of mycobacteriosis was made on the microbiologic exam of bronchoalveolar lavage. Specific treatment was started with contemporary dosage reduction of chemotherapy. After six months of antibiotic treatment the pulmonary lesions improved, whereas CTCL progressed. Therefore, a new antineoplastic regimen was started obtaining control of CTCL, without aggravation of the pulmonary lesions. We highlight the diagnostic and therapeutic pitfalls encountered when pulmonary mycobacteriosis complicates the course and treatment of CTCL.