A 28-year-old woman diagnosed with schizophrenia was admitted to hospital due to progressing dyspnoea for months. At admission, she was oxygen-dependent, and a high-resolution computed tomography revealed ground glass opacities. She had no obvious exposures besides having been treated with lamotrigine for several years. A psychiatrist doubted the diagnosis, and the lamotrigine treatment was tapered. After some months, she became clinically stable without further need of oxygen. Lamotrigine can cause pneumonitis, and this side effect should be suspected in patients who are being treated with lamotrigine and presenting with progressive dyspnoea. Crucial treatment is to remove the exposure.