We interviewed 1,558 adults with idiopathic/cryptogenic epilepsy ascertained from voluntary organizations and 316 of their siblings without epilepsy to determine their personal history of marriage and pregnancy. We examined the effects of seizure type, age at onset, and family history of epilepsy on the sex-specific likelihood of pregnancy in individuals with idiopathic/cryptogenic epilepsy as compared with their same-sex siblings. Overall, men with idiopathic/cryptogenic epilepsy were only 36% as likely as male unaffected siblings ever to have fathered a pregnancy. In men, the reduced likelihood of fathering a pregnancy was associated with partial-onset seizures, early age at onset (< 20 years), and a negative family history of epilepsy, and the effects of these epilepsy characteristics appeared to be mediated through reduced marriage rates. Among men with epilepsy who had ever been married, reproductive disadvantage was confined to those with early-onset (< 10 years) partial epilepsy. Overall, women with idiopathic/cryptogenic epilepsy were only 37% as likely ever to have had a pregnancy as female unaffected siblings; this effect was not strongly influenced by seizure type, age at onset, or family history of epilepsy. In women who had ever been married, unlike in men, reduced likelihood of pregnancy persisted, regardless of seizure type, age at onset, or family history of epilepsy.