We are studying the genetic etiology of schizophrenia in the Republic of Palau, a remote island nation in Micronesia that has been geographically and ethnically isolated for approximately 2,000 years. The first epidemiological phase sought to estimate the lifetime prevalence of schizophrenia and evaluate the familiality of the illness based on complete ascertainment of cases and families segregating schizophrenia. A total of 160 strictly defined cases of schizophrenia were ascertained in a population of 13,750 adults who were 15 years of age and older. The lifetime prevalence of strictly defined schizophrenia in Palau was 1.99% overall and 2.77% in males vs. 1.24% in females. This greater than 2:1 male-to-female risk ratio for schizophrenia was accompanied by an earlier mean age of onset for males (23.3 years) than for females (27.5 years). These 160 cases of strict schizophrenia represent 59 separate families each identified by a single common founder. Eleven of these families have 5 to 14 cases and represent nearly half of the strict schizophrenia cases in Palau. Although schizophrenia is clearly aggregating in these 11 families, cases are distributed sparsely throughout the large sibships. In the entire sample of 160 cases of strict schizophrenia, there were only 11 sib-pairs and 2 sib-trios. When a family was defined to include third-degree relatives, only 11 cases (6.9%) were nonfamilial. The majority of the ascertained cases can be linked together into extended pedigrees with complex mul-tilineal inheritance patterns. These intricately interconnected families may pose challenges for traditional linkage techniques. However, these Palauan families represent a valuable resource for studying the genetic etiology of schizophrenia because there may be fewer susceptibility genes for schizophrenia in this genetic isolate than in the heterogeneous populations that are common throughout the world today. Am.