Objective: To examine the genetic relationships among epilepsies with different seizure types-myoclonic, absence, and generalized tonic-clonic-within the idiopathic generalized epilepsies (IGEs). Background: Careful phenotype definition in the epilepsies may allow division into groups that share susceptibility genes. Examination of seizure type, a phenotypic characteristic less complex than IGE syndrome, may help to define more homogeneous subgroups. Methods: Using the approach that found evidence of distinct genetic effects on myoclonic vs absence seizures in families from the Epilepsy Family Study of Columbia University, the authors examined an independent sample of families from Australia and Israel. They also examined the familial clustering of generalized tonic-clonic seizures (GTCs) within the IGEs in two combined data sets. Families were defined as concordant if all affected members had the same type of seizure or IGE syndrome, as appropriate for the analysis performed. Results: The proportion of families concordant for myoclonic vs absence seizures was greater than expected by chance in the Australian families. In addition, GTCs clustered in families with IGEs to a degree greater than expected by chance. Conclusions: These results provide additional evidence for distinct genetic effects on myoclonic vs absence seizures in an independent set of families and suggest that there is a genetic influence on the occurrence of generalized tonic-clonic seizures within the idiopathic generalized epilepsies. Objective: To examine the genetic relationships among epilepsies with different seizure types-myoclonic, absence, and generalized tonic-clonic-within the idiopathic generalized epilepsies (IGEs). Background: Careful phenotype definition in the epilepsies may allow division into groups that share susceptibility genes . Examination of seizure type, a phenotypic characteristic less complex than IGE syndrome, may help to define more homogeneous subgroups. Methods: Using the approach that found evidence of distinct genetic effects on myoclonic versus absence seizures in families from the Epilepsy Family Study of Columbia University. , the authors examined an independent sample of families from Australia and Israel. They also examined the familial clustering of generalized tonic-clonic seizures (GTCs) within the IGEs in two combined data sets. Families were defined as concordant if all affected members had the same type of seizure or IGE syndrome, as appropriate for the analysis performed. Results: The proportion of families concordant for myoclonic vs absence seizures was greater than expected by chance in the Australian families. In addition, GTCs clustered in families with IGEs to a degree greater than expected by chance. Conclusions: These results provide additional evidence for distinct genetic effects on myoclonic vs absence of seizures in an independent set of families and suggest that there is a genetic influence on the occurrence of generalized tonic-clonic seizures within the idiopathic generalized epilepsies.