The use of fluorescence in situ hybridization (FISH) to detect chromosomal abnormalities has many applications. Use of FISH on archival, paraffin-embedded material has been limited to microscopic sections. We have carried out FISH on preparations of disaggregated nuclei obtained from paraffin-embedded tissue to evaluate chromosome 12 copy number in granulosa-stromal cell neoplasms occurring in infants, children, and adolescents. Trisomy 12 was detected in the majority of cells from three of four juvenile granulosa cell tumors (three ovarian and one testicular) and one malignant granulosa cell tumor. Tetrasomy 12 was observed in a case of ovarian thecoma.