We describe a study of 312 subjects in 71 families near related to a proband with type II hyperprolinaemia. The subjects were Irish travellers (nomads) among whom consanguineous marriage and high fertility are common. Thirteen additional cases of type II hyperprolinaemia were discovered; all were offspring of consanguineous unions. A further 50 subjects were found to have mild hyperprolinaemia. We found a strong association between type II hyperprolinaemia and seizures during childhood but no significant association with mental handicap. Most adults with type II hyperprolinaemia enjoyed normal health and there was no evidence that maternal hyperprolinaemia compromised fetal development. The documented association between type II hyperprolinaemia and seizures may be related to the neuromodulatory or reducing-oxidising effects of proline and pyrroline-5-carboxylate, respectively, that has been shown in vitro. Alternatively, another genetic defect closely linked to the type II hyperprolinaemia allele could be the explanation.