Male subfertility often remains unexplained. Severe intrauterine growth retardation has previously been linked to hypergonadotropic hypogonadism. We examined whether reduced fetal growth, as judged by low birth weight, is associated with unexplained male subfertility later in life. Birth weight and gestational age were obtained by questionnaire from male partners of couples consulting for subfertility, and were transformed into birth weight SD scores. Men with normal semen analysis (n= 128) had a median birth weight SD score of 0.0 (P25-P75range: -0.7 to 1.0), comparable to that of men with explained subfertility (n= 28), and higher (p= 0.012) than that of men with unexplained subfertility (n= 32; median -0.5 SD score; P25-P75range: -0.9 to -0.1). These results extend the link between reduced fetal growth and male subfertility to a range of birth weight that is well within normality. The pathophysiologic mechanism governing this association now remains to be unraveled.