Sarcomatous transformation of a granulosa cell tumor (GCT) is a rare event. We describe the development of a rapidly progressive sarcomatous change in a woman who had initially presented with a classical GCT. A first recurrence occurred 23 months after the initial diagnosis when she was treated with external beam radiotherapy to her pelvis. A second recurrence 76 months following her initial surgery was consistent with a GCT. At 92 months, she presented with a further recurrence, outside of the radiotherapy field. This last recurrence had a different histologic appearance with features of sarcomatous change. Molecular analysis, using both reverse transcription-polymerase chain reaction and complementary DNA microarrays, has been used to analyze tissue obtained before and after the observed change in the tumor. The data show that GCT-specific genes, such as inhibin alpha, estrogen receptor, and follicle-stimulating hormone receptor, have been downregulated in the sarcomatous change. Significant upregulation of genes associated with an inflammatory response was also noted in the sarcoma, and this was consistent with the presence of a marked inflammatory infiltrate seen on histopathology. This study represents the novel application of microarray technology and demonstrates the unexpected finding of expression of the fibroblast activation protein gene in normal ovary. Although tumors such as this may be targets for the novel fibroblast activating protein-directed chemotherapeutic monoclonal antibody sibrotuzumab, the finding of expression in the normal ovary suggests the need for caution.