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SUMMARYThis paper is based on 45 cases in 27 families, all of which have been studied in Children Hospitals. In Switzerland almost all cases ofinfantile amaurotic idiocy (Tay-Sachs)have probably been collected, so that the stated 13 primary and 14 secondary cases may allow an estimation of the frequency of this character, which seems to oscillate temporarily. In the last 10 years no new cases have been observed.All 24 autochthonus Swiss cases are from non-Jewish, mostly rural origin, in which Eastern Jews are not to be supposed as ascendants.Our in all 27 families with TS make Slome's statistic of Jewish and non-Jewish cases in literature much more significant; the incidence of parental consanguinity is, as a matter of fact, nearly twice as high in families with non-Jewish origin.There is no evidence of a milder and longer course of TS in non-Jewish families.The mode of inheritance is, according to Slome,monohybrid autosomal recessive, though in remarkably many sibships there is an accumulation of cases. The reduction method, however, gives a percentage of 28,3 ± 6,2, coming quite near to the expectation of the Mendelian quarter.The penetrance of the character is total, the expressivity generally very equal, with only a few deviations from the well—known clinical picture of TS.The interfamiliar and intrafamiliar variability therefore is small. In not even one of the families studied have there been found other forms of lipoidosis, principally no cases ofsplenohepatomegalia Niemann-Pick. All available respective cases in Switzerland have been collected by the present author and will soon be published in these Acta. In none of these families with M.Niemann-Pickhave cases of M. Tay-Sachs been seen. This fact is liable to prove that these clinically and anatomically very similar conditions derive from independent i. e. different mutations.Modifying genes may account for the incidence of sibships in which all or almost all children died from M. TS.Environmental influences do not seem to favour its manifestations.If in the newer literature from the U.S.A. the Jewish cases of M. TS are still prevalent and if, as we heard, the concomitant manifestations occur fairly often in Israel, the origin may be sought in one at least 200 years old respective mutation in the Jewish population of a relatively restricted White-Russian era.