Neurofibromatosis type 1 (NF1) (von Recklinghausen's disease) is one of the most common human autosomal dominant disorders. In a survey of 374440 17 year old Jewish recruits for military service, 390 cases of NF1 were discovered, with a prevalence of 1.04/1000 (0.94/1000 for males and 1.19/1000 for females), or two to five times the reported prevalence of this disease. NF1 was more common in youngsters whose parents were of North African and Asian origin (1.81/1000 and 0.95/1000, respectively) and less common in those of European and North American origin (0.64/1000). All these differences were statistically significant and may be partially explained by the more advanced parental age of the NF group (as suggested by the larger number of children in the North African and Asian families) or by founder effect or both. The height and weight of the NF1 subjects was significantly lower than those of the controls. The intelligence score of persons with NF1 was similar to that of the control group when corrected for gender and ethnic origin. No significant increase in epilepsy or mental illnesses was found. This study provides evidence of the high overall prevalence of NF1 among Jews in Israel, with significantly increased prevalence in certain ethnic groups.