Geographic patterns of parkinsonism-dementia complex on Guam. 1956 through 1985. Average annual age-adjusted incidence rates of parkinsonism-dementia complex were obtained for the 19 election districts of Guam from 1956 through 1985. The highest rates were found in the southern and central districts, and the lowest rates were found in the northern and western districts. Geographic and temporal patterns of incidence were associated with socioeconomic status but not with geochemical factors. The risk of parkinsonism-dementia complex in susceptible sibships was much higher than that in the general population--even in districts with the highest incidence rates, but especially in districts with the lowest incidence rates. Our evidence tends to support the hypothesis that multiple factors linked to cycad use play an important role in the cause of PDC. Hypotheses related to metal exposure and simple genetic factors were unsatisfactory explanations for the epidemiologic patterns observed. 