We report on a 17-year-old patient with severe bronchiolitis due to Mycoplasma pneumoniae infection. Despite an early 10-day course of clarithromycin, she developed progressive dyspnea, cough, fever, and severe obstructive ventilatory impairment. Sixteen days after onset of the disease a severe hemolytic anemia developed with only cold agglutinins positive at serologic screening. Thoracoscopic lung biopsy revealed diffuse bronchiolitis with suppurative intrabronchiolar inflammation, lymphohistiocytic "cuffing" of the bronchioli, and foam cell aggregates within neighboring alveoli. The infiltrate consisted mainly of CD3+, CD8+ lymphocytes and CD68+ macrophages. The diagnosis of Mycoplasma pneumoniae bronchiolitis was based on repeated complement fixation tests, which turned strongly positive only at day 74 after onset of the disease. Pulmonary function improved slowly under long-term prednisone treatment.