Melphalan can rarely cause interstitial pneumonitis and fibrosis. Although it has been reported previously in patients after conventional doses, we report four cases developing diffuse interstitial pneumonitis (DIP) after high-dose melphalan-based therapy. In a 3-year period, four of 57 (7%) consecutive patients undergoing high-dose melphalan (200 mg/m2; MEL 200) were identified with DIP. Two patients who were heavily pre-treated with alkylators developed progressive respiratory failure despite high-dose steroids and eventually died. The other two patients previously treated with vincristine, adriamycin, and dexamethasone (VAD) improved dramatically on high-dose steroids with complete resolution of their pneumonitis. Melphalan should be added to the growing list of alkylators causing pulmonary toxicity.