THE subject of this article is the disruptive effect of anorexia nervosa on puberty when the illness is of unusually early onset. Twenty girls with anorexia nervosa were investigated and followed up over several years. In all these patients the illness commenced before the completion of puberty, viz, before menarche. Primary amenorrhoea was thus one of the criteria for inclusion in the study. Strong evidence exists that there has occurred a sharp rise in the incidence of anorexia nervosa during recent decades (THEANDER, 1970; KENDELL et al., 1973; JONES et uf., 1980). The illness has also changed in its clinical form, witness the appearance in recent years of bulimic disorders, including bulimia nervosa (RUSSELL, 1979, 1984). The onset of anorexia nervosa is still most often seen during the years of adolescence. But the illness now seems to occur more frequently in mature and older women, either through an earlier illness becoming chronic, or one actually beginning at a later age (DALLY, 1984). Has anorexia nervosa also become commoner in childhood? The answer to this question remains unknown, and is not the subject of this article which addresses itself more to the form of the illness in late childhood and early adolescence. There have been several earlier studies of anorexia nervosa occurring in the very young (LESSER et al., 1960; BL~TZER et al., 1961; TOLSTNP, 1965; WARREN, 1968), but the distinction between the preand post-menarchal onset has not always been clearly made. Those studies which included a follow-up with observations on subsequent physical development have tended to minimize the possibility of any interference with the process of puberty (WARREN, 1968; SILVERMAN, 1974). In a preliminary communication on 9 patients whose illness began at a critical phase of the pubertal process and before the menarche, it was suggested that puberty might be delayed and that in some patients there might even occur some permanent failure of growth in stature and in breast development (RUSSELL, 1983). In the present study the series of patients has been extended to 20, and the follow-up is more complete. It is now possible to report in more definite terms on the impact of anorexia nervosa on the sequence of pubertal events when the illness begins before the menarche and especially when it follows a chronic course.