Of more than 3000 cases of surgically removed pituitary adenomas, 11 were defined as "double adenomas," i.e., 2 morphologically or immunocytologically distinct tumors. In 8 cases, the lesions exhibited differing histological features and immunophenotypes; in 2 specimens, distinct ultrastructural features were noted as well. In another instance, despite histological and immunocytological uniformity, the two neoplastic components demonstrated distinct ultrastructure. In yet another case, the two adenomas were consecutively removed; despite similar histological features, they differed in immunocytological and ultrastructural characteristics. Last, in one case, the adenoma was histologically uniform, but a portion of the mass exhibited immunoreactivity by ultrastructural features distinct from those of the remainder of the lesion. Hormonal excess attributed to both tumors could be correlated with endocrine manifestations in two cases. Double adenomas of the pituitary occur infrequently. In routine histological sections of surgical material, they are often difficult if not impossible to identify. Presented herein are clinical and endocrinological data on 10 cases of double pituitary adenomas correlated with morphological and immunocytochemical results. The literature regarding multiple adenomas is reviewed as are the diagnostic and therapeutic difficulties associated with these rare lesions.