<qa>  What is the most important concern about side by side method?Malignant obstructive jaundice is a common complication of advanced stage cholangiocarcinoma, GB cancer, and pancreatic cancer. In biliary stricture by malignancy, biliary drainage with placement of self-expanding metal stent SEMS for palliation is the therapy of choice in these patients. On hilar biliary malignant obstruction, bilateral stent used to be inserted. There are two kinds of bilateral metal stent insertion methods in hilar biliary malignant obstruction. Stent in stent vs. side by side Previously, two studies was conducted to show the superiority of bilateral metal stent insertion between stent in stent and side by side. Biliary drainage and stent patency was a little superior in side by side than stent in stent. But, complication related with bilateral metal stent was increased in patients with side by side method. Another study showed that there was no significant difference between those bilateral stent insertion methods. The most important concern about side by side method is associated with portal vein thrombosis by bilateral stent diameter. Selection of proper stent insertion method in patients with hilar malignant biliary obstruction is still controversial. 