A study on possible association of maternal factors with undescended testis was undertaken. A comparison was made using mothers of boys with the disease (No. = 108) and mothers of boys without the disease (No. = 108). The boys without the disease were selected from outpatients by individually matching the birth year with the case. A smaller proportion of mothers having babies with the disease experienced vomiting during the index pregnancy (the estimated relative risk RR = 0.51, P = 0.02). A larger proportion of them had delivered by vacuum extractor delivery, cesarean section, or breech extraction (RR = 2.10, P = 0.04). A higher proportion of cases were complicated with inguinal hernia (RR = 9.00, P = 0.03), congenital cardiac diseases (RR = infinity, P = 0.008), or other various kinds of congenital disorders. A larger proportion of these mothers had never breastfed for the cases (RR = 3.75, P = 0.02). Exposure to external estrogen in the utero was not noted to be associated with undescended testis.