Febrile seizures are the most common form of seizures, occurring in an estimated 2-5% of North American children. We carried out a systematic pedigree study of febrile seizure probands. Forty of 52 probands (77%) in a referral population selected for increased severity had more than one case per family: one family had 10 cases, one family had 7, 3 families had 6, 2 had 5, 3 had 4, 13 had 3, and 17 had 2 cases. Mode of inheritance in the multicase families best fit the hypothesis of autosomal dominance with reduced penetrance. Polygenic inheritance could not be excluded for some of the smaller families. There was no support for X-linked or mitochondrial inheritance. Penetrance was calculated to be 0.64. Because the cases were selected for increased severity, this represents a useful estimate of the upper limit of penetrance and is in agreement with twin studies. Simulated lod scores showed adequate power for a linkage study in the absence of heterogeneity. Individual families had simulated average lod scores as high as 2.1. However, with potential heterogeneity, assuming only 70% of families share the same disease locus, average lod scores were marginal, and a high density map of marker loci and additional families would be required to document linkage.