We analyzed by restriction mapping the genomic organization of the estrogen receptor gene in several primary human brain tumors in order to investigate the possible relationships between the development of these tumors and gonadal steroid hormones. In 5 out of 23 meningiomas the Eco RI restriction of genomic DNAs revealed 6 invariant normal fragments of 6.5, 4.8, 3.8, 3.1, 2.7 and 1.7 kb, plus 2 additional variant fragments of either 8.6 or 2.5 kb. More detailed analysis showed that these variant bands hybridized with a probe specific for the middle region of the estrogen receptor cDNA, coding for the DNA-binding domain of the receptor. This abnormal restriction pattern was found only in these meningiomas and not in other brain tumors or DNAs obtained from peripheral blood lymphocytes.