We used magnetic resonance (MR) imaging to evaluate treatment response of 10 consecutive lung cancer patients while they were receiving radiation therapy. Patients were scanned before treatment, during treatment, at completion of treatment, and if possible, at 3-month intervals thereafter. The initial tumor response to radiation was increasing signal intensity and increasing heterogeneity, best seen on T2-weighted images. Small tumors virtually disappeared, whereas larger masses remained as complex cystic structures or developed cavities. The adjacent irradiated lung parenchyma had increased signal on both the T1− and T2-weighted images as early as 17 days after start of treatment. The signal intensity continued to increase for several months after treatment, but subsequently decreased.