DURING the years 1941 to 1945 there occurred a most striking epidemic of thyrotoxicosis in Denmark. The epidemic was described and discussed by Meulengracht1 while it was at its climax, and it has since received a detailed analysis by Kurt Iversen,2 who has published a monograph on it. The cause of the epidemic is unexplained. As Means has so aptly expressed it to me, it remains for the time being as "just a brutal fact" which calls for an explanation. It may perhaps be expedient, as the starting point for new investigations, to set down some epidemiologic facts regarding this temporary rise in the frequency of the disease and about the complaint itself. EARLIER OBSERVATIONS ON THE EPIDEMIC OCCURRENCE OF THYROTOXICOSIS In Iversen's recently published monograph there is an extensive review of the literature relating to temporary rises in the frequency of thyrotoxicosis. It appears from this that