From 1992-2004, single lung transplantation has been performed in seven patients with end-stage pulmonary sarcoidosis at the Danish National Centre for Lung Transplantation. The objective was to assess whether recurrent sarcoid granulomas in the lung graft are derived from recipient or donor immune cells. Three patients had sarcoid recurrence in the lung graft, but none had clinically overt extra-thoracic sarcoidosis. Graft sex-mismatch was present in one patient, a 52-yr-old female having a lung graft from a male donor. In order to discriminate between recipient and donor cells fluorescence in situ hybridisation (FISH), using probes for both X- and Y-chromosomes, was applied on transbronchial lung biopsies (TBB) from the lung graft containing sarcoid granulomas. The recipient's explanted lung contained multiple active sarcoid granulomas. TBB from the implanted donor lung 5 months after transplantation showed sarcoid granulomas. FISH showed that the immune cells in the granulomas were X-chromosome positive and Y-chromosome negative and, therefore, were derived from the recipient. In conclusion, the results indicate that recurrent sarcoid granulomas in the transplanted lung are derived from recipient's immune cells, having colonised the lung allograft.