Introduction Iatrogenic lung injury is a very rare, albeit serious complication with antineoplastic therapy, including immunomodulatory drugs. Pneumonitis typically presents clinically with symptoms such as cough, dyspnea, fever, and hypoxemia. Radiographic evaluation often demonstrates diffuse, patchy infiltrates and ground-glass opacities. Case report We present a case in which therapy from an immunomodulatory drug, lenalidomide, elicited a pneumonitis in the form of a 9 cm lung mass. An exhaustive workup was completed to rule out viral, bacterial, and fungal infections as well as malignant causes. Lenalidomide-induced lung injury was suspected. Management and outcome Lenalidomide was discontinued and corticosteroid therapy was initiated. This resulted in a complete clinical and radiographic resolution of symptoms. Discussion Several case reports of pneumonitis have been associated with immunomodulatory drug therapy, and while most of these exhibit diffuse ground-glass opacities radiographically, our patient presented with a 9 cm lung mass. Our findings stress the importance of a thorough medication review while ruling out other potential causes of lung injury.