Undescended testis [UT] is the most frequent disorder of male sexual differentiation affecting 0.8% of all boys by age 1 year. To establish factors [genetic or environmental] associated with UT and to estimate heritability, 261 cases with UT were studied. For comparison, 506 normal control groups were randomly selected. The frequency of associated congenital abnormalities among patients with UT was significantly higher than among control = 21.49, P≤ 0.000]. The frequency of first born boys was also significantly higher in the group with UT than in the control group [X2 = 19.8; p ≤ 0.01]. A positive association was found for low birth weight [OR = 3.31; Cl: 1.97-5.55], and complicated deliveries [OR = 2.72; CI: 1.5-4.67]. Maternal risk factors; threatened abortion [OR = 3.4; Cl: 1.64-7.09], perinatal morbidity [OR = 3; Cl: 1.42-2.35] and chronic diseases [OR = 3.05; CI: 1.37-6.85] were correlated with UT. No significant association was found for gestational age, parental consanguinity, parental ages, exposure of mothers to radiation and oral contraceptives. The occurrence of UT could be confirmed in 1.2% of fathers. The recurrence risk for brothers was 5% [an empirical risk of about 1 in 20]. The value of heritability for the first degree male relatives was 62%, suggesting the role of genetics in the etiology of UT. Determining the etiology of UT is an important step towards primary prevention of this common birth defect.