A 50-year-old female developed a cyst of the iris stroma in her right eye. Because of slight enlargement within 10 weeks sector iridectomy was performed. Histology showed that the cyst was lined by a cellular layer resembling conjunctival or corneal epithelium. The benign clinical course (normal function, normal intraocular pressure and no recurrence after 4 years) and the intrastromal rather than epistromal localization hold evidence for a spontaneous origin. Spontaneous cysts of the iris stroma especially in adults are rarely described in the literature. The question still to be answered is whether these lesions are really spontaneous in origin or whether they represent only an intraocular epithelial invasion due to an occult perforation of the eye.