When a prenatal diagnosis of Klinefelter syndrome (KS) is made, a couple is faced with an unfamiliar and unexpected diagnosis. The aim of this article is to give clues to prenatal counselling in this situation. The information provided to couples facing a prenatal diagnosis of KS should ideally be based on longitudinal studies of unselected individuals, including those diagnosed prenatally. Indeed, there are several reasons to think that the phenotype of individuals diagnosed prenatally is globally less severe than in those diagnosed postnatally. Based on these studies, the evidence to be explained to couples to help them make an informed decision about the pregnancy is the following: except for rather tall height, generally normal appearance throughout life; increased risk of learning disabilities; spontaneous puberty, reduced testicular size, usual need for testosterone supplementation from adolescence onward; increased risk of gynecomastia; sexual orientation similar to the general male population; infertility, but with the possibility of having biological offspring with assisted reproductive techniques. In this article, we review the evidence about the phenotype of KS according to the circumstances of diagnosis and its use in counselling couples faced with a prenatal diagnosis of this common condition. CONCLUSION: Cohort studies including individuals with KS diagnosed prenatally are still lacking.