A 13-month-old Korean female presented with Cushing disease and diabetes insipidus. On MRI, a 3.5-cm, focally cystic, contrast-enhancing, sellar and suprasellar mass was noted. Aside from blood adrenocorticotropin (ACTH) and cortisol elevation, other pituitary hormone blood levels were normal or markedly reduced. The subtotally resected lesion consisted of synaptophysin-immunoreactive lobules of (a) large, polygonal, amphophilic, PAS-positive cells immunoreactive for ACTH, beta-endorphin, alpha melanocyte stimulating hormone (MSH), and keratin (CAM5.2) in some cells showing Crooke hyaline change, (b) less frequent acidophilic, growth hormone (GH) immunoreactive cells, and (c) rare luteinizing hormone (LH) and/or alpha subunit immunopositive cells. Also conspicuous were smaller cells resembling Rathke-type epithelium forming rosettes to sizable glands immunoreactive for EMA, keratin, S-100 protein, galectin-3 and rarely for synaptophysin and/or one of the above-noted adenohypophysial hormones. Transcription factors, including Neuro-D1 and Pit-1, were present in ACTH- and GH-producing cells, respectively, but only in occasional Rathke-type cells. The MIB-1 labeling index (LI) was 1.5% in secretory cells and 39% in Rathke-type epithelium. Ultrastructurally, the tissue resembled fetal pituitary of 10-12 weeks gestation and contained fully differentiated corticotrophs and somatotrophs, scant cells of glycoprotein-hormone producing type with small secretory granules, and glandular epithelial cells consistent with committed, but largely undifferentiated Rathke-type epithelium. We consider the tumor as a pituitary blastoma, a lesion composed of multiple cell types common to the development of the affected organ based upon (a) prominence of primitive Rathke-type epithelium, (b) disposition of secretory cells in lobules rather than acini, (c) the limited range of secretory cells represented, (d) the presence of their corresponding transcription factors, and (e) ultrastructural features indicating orderly development of the 10- to 12-week embryonic stage.