Over the last 60 years an abnormally high prevalence of atypical Parkinsonism has been reported in 5 different geographic isolates. It was first described on Guam, later in New Guinea and in the Kii peninsula, on Guadeloupe, and in New Caledonia. We investigated the phenotype of atypical Parkinsonism in three of these foci and observed several similarities with dementia in most and amyotrophic lateral sclerosis in some. This disappearance of this disease in two places--Guam and New Guinea--suggested an environmental origin which has not been clarified before the disease ended. The exposure to annonaceae acetogenins and/or rotenone has been documented in four of these places, and experimental studies in animals demonstrated annonaceae acetogenins neurotoxicity, which is similar to rotenone neurotoxicity. Simultaneous exposure to acetogenins and rotenone could produce a synergistic toxicity on neurons.