Follicular neoplasms of oncocytic type in the thyroid gland frequently cause diagnostic problems and prognostic uncertainties. To identify numerical chromosomal aberrations of possible pathogenetic importance, we determined chromosome copy numbers in situ in interphase nuclei of 31 oncocytic adenomas and 25 oncocytic carcinomas. Archival formaldehyde-fixed, paraffin-embedded tumor samples and normal control thyroid tissues were arranged in arrays and analyzed by fluorescence in situ hybridization (FISH). We used pericentromeric or locus specific probes for chromosomes 1, 7, 8, 9, 11, 12, 17, 18, 22, and X as well as for the oncogenes Her2/neu, cyclin D1, N-myc, and c-myc. The average number of aneusomies per nucleus was significantly higher in carcinomas than in adenomas, and in both, monosomies were more frequent than polysomies. Loss of chromosome 22 was found in 8 of 21 (38%) carcinomas; in 5 cases, it was associated with chromosome 2 monosomy. Conversely, chromosome 2 aberrations were not found in adenomas. Monosomies for chromosome 8 and X were detected in most adenomas and carcinomas. The most common gains in adenomas and carcinomas were for chromosome 7 (13.8% and 32.0% of the cases, respectively), chromosome 12 (9.6% and 12.0%), and chromosome 17 (19.3% and 32.0%). None of the adenomas with trisomy 17 was associated with gains for chromosomes 7 and 12. None of the analyzed oncogenes was found to be amplified by FISH analysis. Our results indicate that numerical chromosomal aberrations in oncocytic follicular tumors of the thyroid gland are common findings and suggest that different patterns of aberrations may occur in these neoplasms.