BACKGROUND: Human metapneumovirus (hMPV), a recently discovered respiratory virus, is associated with clinical disease in young and elderly persons. OBJECTIVE: To determine the importance of hMPV in hematopoietic stem-cell transplant recipients. DESIGN: Retrospective survey of patients with consecutive residual bronchoalveolar lavage (BAL) samples. SETTING: Referral cancer center. PATIENTS: Hematopoietic stem-cell transplant recipients who underwent BAL because of lower respiratory tract disease. MEASUREMENTS: Bronchoalveolar lavage specimens were assayed by quantitative real-time polymerase chain reaction methods. RESULTS: Human metapneumovirus was detected in BAL specimens from 5 of 163 patients (3.0%). Persistent viral infection was noted in 3 patients with several samples, and hMPV was detected in 1 of 2 lung specimens tested. Infected patients became symptomatic within the first 40 days after transplantation. Initial symptoms included fever, cough, nasal congestion, and sore throat. Clinical findings included respiratory failure, pulmonary hemorrhage, and culture-negative septic shock. Four of 5 patients died with acute respiratory failure. LIMITATIONS: This retrospective study did not evaluate asymptomatic patients or those with mild disease. CONCLUSION: Human metapneumovirus infection in the lower respiratory tract is associated with respiratory failure in immunocompromised adults who were previously considered to have "idiopathic pneumonia." The infection may result in fulminant respiratory decompensation and shock after transplantation.