A 60-year-old woman under treatment with letrozole for metastatic breast cancer underwent 18F-FDG PET-CT for restaging. A new widespread intense splenic FDG uptake without nodular lesions and multiple FDG-avid mediastinal and abdominal nodes were observed. Based on these findings, a nodal and transbronchial lung biopsy was performed. Histological results were compatible with sarcoidosis. The patient began steroid treatment and 6 weeks after a PET-CT showed normalisation of both splenic and nodal uptake. In our case, 18F-FDG PET-CT has been useful in detecting a rare case of splenic sarcoidosis without typical nodular lesions on CT images, impacting the patient’s treatment and prognosis.