We report a series of four patients who presented with complaints of diffuse non-scarring alopecia. They had similar clinical features of alopecia, hyperseborrhea, and distinct keratinaceous hair casts that encircled the hair shafts. Propionibacterium acnes was isolated from two of the patients' scalp, and Gram-positive, Giemsa-positive bacteria were seen in the hair follicles in the scalp biopsy of one of the patients. The patients' symptoms did not respond to standard treatment for seborrheic dermatitis, but responded to a course of systemic antibiotics targeting P. acnes. We propose a role for P. acnes colonization of the terminal hair follicles in the pathogenesis of hair casts, and possibly diffuse non-scarring alopecia. Possible mechanisms of pathogenesis are discussed with a literature review.