ABSTRACT.In order to clarify the pathogenesis of rickets in preoperative patients with extrahepatic biliary atresia, we evaluated baseline serum 25‐OHD and 1,25(OH)2D levels and correlated serum 25‐OHD levels with increase in age and season of birth in 16 preoperative patients. Further, parenteral vitamin D2tolerance tests were performed in 5 cases. Serum 25‐OHD and 1,25(OH)2D levels were significantly lower than those in 15 normal controls. There was a negative correlation between the serum 25‐OHD levels and increase in age. The patients born during the winter had lower serum 25‐OHD concentrations than those born in summer. The mean value of increased 25‐OHD levels after the parenteral vitamin D2tolerance tests did not differ from that of 6 controls. Since there was no impairment of vitamin D 25‐hydroxylation, the reduction in serum 25‐OHD may therefore be mainly due to disturbed intestinal vitamin D absorption. It was also concluded that season of birth and increase in age are pathogenic factors in the etiology of rickets in preoperative patients with extrahepatic