Endodermal sinus tumor (EST) of the ovary is a germ cell tumor that is now recognized more frequently than in the past. Since problems concerning natural history, prognosis and therapy of these neoplasms are not entirely resolved, it should be underlined the need to report new cases. The authors describe four cases of EST with reference to serial alphafetoprotein determination by radioimmunoassay; they also emphasize the importance of this technique as a very sensitive method in monitoring therapy and in detecting recurrences of disease even before any clinical evidence.