Although pituitary adenomas are among the most frequent intracranial neoplasms, only very few have been cytogenetically analyzed. We have short-term cultured and karyotyped 28 consecutive pituitary adenomas (16 clinically nonfunctioning adenomas and 12 clinically functioning adenomas), finding a normal karyotype in 22, whereas 6 had clonal chromosome aberrations (5 nonfunctioning pituitary adenomas and 1 prolactinoma). The abnormal karyotypes were relatively simple. Most anomalies were numerical, with a structural rearrangement, t(6;19), being found in only one tumor. The most common aberrations were trisomy 7 (3 adenomas), trisomy 9 (2 adenomas), trisomy 12 (2 adenomas), trisomy 20 (2 adenomas), and loss and gain in 2 separate clones of one X chromosome (2 adenomas).