It has recently been shown that most cases of clinically overt erythropoietic protoporphyria (EPP) result from coinheritance of a mutated ferrochelatase gene and a commonly occurring low-expression normal variant allele. The identification of two polymorphic variant sequences associated with this low-expression allele now enables improved predictive counselling for couples where one partner has EPP. We describe a patient and his spouse in whom we have used such genetic analysis to provide an accurate estimate of the chance that their future offspring may suffer from EPP.