Chromosome 11 abnormalities were detected by fluorescence in situ hybridization (FISH) technique and compared with DNA ploidy in 24 surgically removed pituitary adenomas. The tumors were diagnosed and classified by histology, electron microscopy, and pituitary hormone immunocytochemistry. They included 2 densely granulated somatotroph (DG-SM) and 4 sparsely granulated somatotroph (SG-SM) adenomas, 3 SG lactotroph (LT), 2 mixed somatotroph-lactotroph (SM-LT), 4 functioning corticotroph (CRT), 1 silent CRT subtype 1, 1 thyrotroph, 1 mixed thyrotroph-somatotroph, 2 gonadotrophs, and 4 null cell adenomas. FISH analysis with an alpha-satellite DNA probe specific for chromosome 11 showed numerical abnormalities in 16 functioning (94%) and 5 nonfunctioning (71%) adenomas. Ten functioning tumors showed aneuploid histograms, whereas the remaining and all nonfunctioning adenomas were diploid. Aberrant chromosome 11 signals were noted mostly in aneuploid adenomas involving 17% to 100% of their cell population. The severity of chromosome 11 aberrations in adenomas containing extra copies often correlated with a higher DNA index (DI). Monosomy 11 as dominant aberration was noted in a mixed SM-LT and to a lesser degree in 3 CRT adenomas involving 21% to 97% of their cell population. Two of these CRT adenomas were associated with normal DI, whereas the remaining third showed a high DI, indicating increased copy number of chromosomes other than of chromosome 11. In conclusion, chromosome 11 abnormalities are common in all types of pituitary adenomas, occurring more frequently in functioning tumors. Specific numerical abnormalities, such as monosomy and trisomy, tend to be associated with certain adenoma types, whereas tumors with extra chromosome 11 copies often exhibit aneuploid histograms.