Endo- and exo-cycloisomerizations of 4-pentyn-1-ol have been studied computationally with density functional theory, in conjunction with double-zeta and triple-zeta basis sets, both in the absence and in the presence of tungsten carbonyl catalyst. In the absence of the catalyst, both endo- and exo-cycloisomerizations have been calculated to have a very high activation barrier of similar to50-55 kcal/mol and cannot take place. With tungsten pentacarbonyl catalyst, endo-cycloisomerization becomes a complex multiple-step reaction and proceeds with a rate-determining barrier of 26 kcal/mol at the C-alpha-C-beta hydride migration step to form a vinylidene intermediate. The primary role of the tungsten catalyst is to stabilize the vinylidene intermediate, thus lowering the rate-determining barrier. The second important role of the tungsten catalyst in endo-cycloisomerization is to assist the OH hydride migration to C-alpha by making it a multistep process with small activation barriers. The exo-cycloisomerization with the catalyst still has a high rate-determining barrier of 47 kcal/mol. These findings clearly explain the experimentally observed endo- selectivity in the cycloisomerization of 4-pentyn-1-ol derivatives and support the experimentally proposed mechanism.