About 50%of intersex cases are due to male pseudohermaphroditism, and of thes e cases, 50%are not clarified aetiologically. The association of idiopathic mal e pseudohermaphroditism and prenatal growth retardation has been recently report ed. The aim of this study was to verify whether there was a difference in weight and/or length at birth between idiopathic and nonidiopathic male pseudohermaphr oditism patients. A total of 70 patients with male pseudohermaphroditism were re cruited; 35 non-idiopathic and 35 idiopathic. Birth weight and length were conv erted to z scores, and the severity of genital ambiguity was classified accordin g to Prader grades: less virilised (Prader 1 to 3) and more virilised (Prader 4 or 5). Data were analysed using a Mann-Whitney test, odds ratio and logistic re gression analysis. Birth weight ( P =0.028) and length ( P =0.01) z scores were lower in the idiopathic male pseudohermaphroditism gr oup compared to the non-idiopathic group and were also significantly decreased among the less virilised patients, both in the sample as a whole (weight z score , P =0.002; length z score, P =0.0008) and in the group of idiopathic patients ( weight z score, P =0.013; length z score, P =0.007). According to logistic regre ssion analysis, only birth length z score significantly predicted the severity o f the genital ambiguity in patients with idiopathic male pseudohermaphroditism ( P =0.0007). Conclusion:There is an association between prenatal growth retardat ion and male pseudohermaphroditismwhich may be due to genetic factors not clarif ied yet or to environmental factors which act early in gestation. The association of idiopathic mal e pseudohermaphroditism and prenatal growth retardation has been recently reported ed. The aim of this study was to verify whether there was a difference in weight and / or length at birth between idiopathic and nonidiopathic male pseudohermaphroditism patients. A total of 70 patients with male pseudohermaphroditism were re cruited; 35 non-idiopathic and 35 idiopathic. Birth weight and length were convokedrted z scores, and the severity of genital ambiguity were classified accordin g to Prader grades: less virilised (Prader 1 to 3) and more virilised (Prader 4 or 5). Data were analysed using a Mann-Whitney test, odds ratio and logistic re Birth weight (P = 0.028) and length (P = 0.01) z scores were lower in the idiopathic male pseudohermaphroditism gr oup compared to the non-idiopathic group and were also significa both of the less virilised patients, both in the sample as a whole (weight z score, P = 0.002; length z score, P = 0.0008) and in the group of idiopathic patients (weight z score, P = score, P = 0.007). According to logistic regre ssion analysis, only birth length z score significantly predicted the severity of the genital ambiguity in patients with idiopathic male pseudohermaphroditism (P = 0.0007). Conclusion: There is an association between prenatal growth retardation and male pseudohermaphroditismwhich may be due to genetic factors not clarified yet or to environmental factors which which act early in gestation.