Penetrance of schizophrenia and related disorders was calculated in 27 multiplex pedigrees ascertained by a consistent set of screening and selection criteria. The rationale for the study was that single major locus linkage models are frequently used on a pragmatic basis to analyze data for schizophrenia which is most likely to have a polygenic mechanism. Penetrance estimates assuming Mendelian inheritance represent maximum values and thus can provide guidance for construction of appropriate linkage models. Four diagnostic models were considered: narrow (schizophrenia and chronic schizoaffective disorder), intermediate (including other non-affective psychoses), broad (including schizotypal and paranoid personality disorders), and broad + suspected (including suspected schizophrenia spectrum disorders). Penetrance was calculated in the youngest affected adult sibship, under both dominant and recessive inheritance assumptions, either without correction, or with a correction that excluded individuals necessary to meet pedigree selection criteria. Without correction, penetrance values ranged from 0.70 to 0.90 assuming dominant and 1 .O to > 1 .O assuming recessive inheritance. After correction, the ranges were 0.30-0.5 1 for dominant and 0.47-0.59 for recessive models. The corrected values are likely to be overestimates given that the penetrance of any one locus in a multilocus model must be lower. It is suggested that lod score analyses of schizophrenia should attempt to derive information primarily from affected