Stanhope, J. M. (Epidemiology Unit, Wellington Hospital, Wellington, New Zealand), Brody. J. A., and Morris, C. E. Epidemiologic features of amyotrophic lateral sclerosis and parkinsonism-dementia in Guam, Mariana Islands. mt. J. EpId. 1972, 1: 199–210. Amyotrophic lateral sclerosis and parkinsonism-dementia continue to occur at extraordinarily high rates among the native people of Guam. From 1950 to 1969 the ALS rate per 100, 000 for males was 108, and for females was 42 and the disease accounted for 15 per cent of all adult mortality for males and 11 per cent for females. The mean age at death was approximately 50 years with approximately 30 per cent of patients surviving 5 years after onset and 12 per cent surviving 10 years. Clinically the disease resembles classical ALS seen elsewhere but the rates are 50 to 100 times higher. Parkinsonism-dementia is apparently unique to Guam. From 1960 to 1969 the rate per 100, 000 for males was 115 and for females 32. The mean age at death was approximately 60 years, with 35 per cent surviving for 5 years after onset and only one person surviving 10 years. PD accounted for 15 per cent of deaths in adult males and 8 per cent of deaths in adult females. The rate of ALS appears to be declining somewhat while PD has not been observed for long enough to detect distinctive trends. The diseases occur essentially in individuals with Chamorro (native Guamanian) ancestry, although in 11 instances one parent of a case was non-Chamorro. About 10 per cent of cases were conjugal. No distinctive patterns of Mendelian inheritance have emerged although the diseases tend to occur to some extent in familial clusters. Geographic patterns have emerged, and while most of the island has intermediate rates, certain villages have consistently high or consistently low rates. Studies of possible toxic or viral agents are in progress but are thus far negative. The relationship between ALS and PD is not clear, and while there is some clinical and pathologic overlap, for the most part the two syndromes are distinctive. Recent neurochemical and neuropathologic findings suggest that some abnormalities occur in a high percentage of Guamanians without overt ALS or PD and when sufficient material is accumulated it may shed light on the observed geographic and familial patterns.