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The isonymy method of estimating inbreeding is explored as a research strategy in the genetic epidemiology of noninsulin-dependent diabetes mellitus in an island population. Vinalhaven Island, Maine, was the site of a major community-wide study of diabetes in 1965-70, including glucose tolerance testing (GTT) of over half of the adult population in 1966-67 and complete pedigree collection on all island natives. Final analysis was never accomplished due to the principal investigator’s death, but published preliminary findings noted a 40% diabetes prevalence and very high inbreeding. Results of the present study show that the prevalence of GTT’s in the imparied glucose tolerant and diabetic ranges combined averages 19% in natives and 16% in nonnatives of the 1966-67 screening population. Total inbreeding estimated by isonymy is low, 0.0034 (0.0017 after correction for polyphyletic surnames). The pedigree F for screening population sibships with at least one member testing in the impaired glucose tolerant or diabetic ranges is even lower, 0.0003. Isonymy is judged to overestimate inbreeding, but to be of some use when applied cautiously to specific problems.