A 70 years old man presented with asthenia, dyspnea and dry cough since a few days. His medical history included primary myelofibrosis (MF) diagnosed 1.5 month earlier, treated with HU since 1 month. The patient had stopped the HU-treatment 4 days before because he suspected a causative association with the HU-therapy. CT-scan ruled out pulmonary embolism but showed apical predominant patchy ground-glass opacities. Elevated inflammatory markers were found in the serum (CRP 155 mg/l). Bacterial pneumonia was suspected and antibiotic treatment with Co-Amoxicillin and Clarithromycin was started with prompt remission. On discharge, the therapy with HU was resumed. 2 days later the Patient returned with aggravation of the symptoms. Chest X-ray showed pronounced infiltration of the upper lobes with small bilateral pleural effusions. Neither in the blood culture nor in the bronchoalveolar lavage (BAL) an infectious cause was found. BAL showed a picture compatible with hypersensitivity pneumonitis. HU was stopped immediately and treatment with prednisolone 40 mg was initiated. With this therapy the symptoms regressed quickly.  Only a few cases (< 20) of HU induced pneumonitis have been described in the literature. The signs and symptoms of HU induced pneumonitis are initially mostly misinterpreted as a bacterial pneumonia.  Most important is the immediate cessation of the HU-therapy. There exists no recommendation regarding corticosteroid treatment.