Immunoglobulin G4 (IgG4)-related disease is a relatively rare and only recently recognized immune-mediated fibro-inflammatory condition that is commonly associated with autoimmune pancreatitis. Reports have further been characterized in almost all other organ systems, with several lung-related IgG4 disease reports emerging over the past decade. IgG4-related disease affects more than one organ in 60-90% of patients. To this date, there have been few published cases of pathologically proven isolated IgG4-related lung disease (IgG4-RLD), where no other organ is affected. We report an isolated pulmonary case of IgG4-RLD in a 65-year-old female with clinical and radiological manifestations suggestive of primary lung malignancy. CT revealed multiple sub-solid ground glass opacities, several of which were part-solid, others were pure ground glass. Histological analysis revealed IgG4 disease with no evidence of neoplasia. Serum IgG4 levels were elevated (206 mg dl-1). Malignancy was ruled out and the patient was treated with corticosteroids, though there was no change in CT appearance over 16 months. The CT imaging pattern in our case is atypical from previous literature characterisation.