To establish the unique and common clinical and microscopic characteristics of the alopecias associated with vitamin D-dependent rickets (VDDR) type IIA and with hairless gene mutations.Design: A comparative clinical, histologic, and immunohistochemical study of the alopecias in 6 patients with VDDR IIA and 4 patients with atrichia with papular lesions (APL) and/or alopecia universalis congenita (AUC) (hereinafter "APL/AUC").Main Outcome Measures: Clinical data were gathered from medical records, personal interviews, and physical examinations. Histologic and immunohistochemical studies were performed on 6 scalp punch biopsy specimens from each of the 2 studied groups. Results:The alopecias in VDDR IIA and APL/AUC showed similar clinical, histologic, and immunohistochemical features. The clinical presentation of the VDDR alopecia resembled either the APL phenotype (ie, with papules and milia) or the AUC phenotype (without pap-ules and milia). The main histologic findings included void infundibula; absence of the lower two thirds of the hair follicles, often replaced by vertically oriented irregular epithelial structures or epithelial cysts; irregular epithelial structures, often with small cysts in the middle and lower dermis; and small, medium, and large keratinizing cysts at all levels of the dermis. The larger epithelial cysts in the upper dermis stained positively for cytokeratin (CK) 10, which suggests an infundibular derivation, whereas the remaining irregular epithelial structures and cysts in the middle and lower dermis stained positively most frequently for CK17, CK19, and CD34, which suggests an outer root sheath derivation. Conclusions:The alopecias associated with VDDR IIA and with hairless gene mutations show striking clinical and microscopic similarities. Disintegration of the lower two thirds of the hair follicles seems to be the underlying defect, and a common pathogenetic pathway might be involved.