Adult hallux varus is an uncommon clinical entity usually caused by an inflammatory arthropathy or overcorrection during bunion reconstruction. We present five cases of unexplained spontaneous hallux varus. Clinically, all patients initially were found to have flexible deformities, and no evidence of underlying inflammatory disease or history of trauma. Symptoms were easily relieved with shoe wear modifications. Two of the patients developed more rigid deformities and subsequent medial callusing of the first toe and varus deformities of the lesser toes. These two patients eventually required surgical intervention. A Keller arthroplasty was performed on the first metatarsophalangeal joint and valgus proximal phalangeal osteotomies were done on the lateral toes when indicated. The outcome for these patients was good. Possible etiologic mechanisms are discussed. It is postulated that in these cases, the abductor hallucis tendon inserts more medially on the phalanx and overpowers the adductor. This produces a medially directed moment and, with time, a varus deformity.