The authors describe histologic and immunohistologic features of seven cases of ovarian mature teratomas showing multiple cysts, optically empty, in their walls. These cysts have been detected in 7 out of 31 cases of ovarian teratomas observed from 1985 to 1991 (incidence rate 22%); they measured from less than 0.1 mm up to 5 mm in their largest size, and were lined by flattened cuboidal cell, histiocytes and multinucleate foreign body-type giant cells. Masson trichrome stain revealed a fibrous framework around them. Immunohistochemical stains provided evidence of both macrophagic and endothelial nature of the lining cells of the cysts because of their cytoplasmic positivity for KP1, KIM 6 and, though focally, for FVIII-related antigen. Ovarian teratomas exhibiting such aspects, defined as pneumatosis cystoides-like appearances or multicystic structures, have been previously described. These peculiar morphologic finding are reported in gynecological literature under the heading of sieve-like areas but, generally, little information about the mechanisms of their constitution is available. Etiopathogenic hypotheses are hereafter considered and discussed. The present findings suggest that these multicystic spaces may be the result of a granulomatous lipophagic reaction due to extravasated material (such as lipids or sebum) and/or of a dilatation of lymphatics vessels.