Cyclization of 6-octen-1-als with Rh(I) (Wilkinson) complex was examined. In all cases with methyl substituents at the C2- or C3-position, a mixture of cis- and trans-cyclohexanol derivatives was obtained, but 7-methyl-6-octen-1-al without any substituents instead underwent a decarbonylation reaction. 4-Oxa-analogues also afforded a similar result. In Rh(I)-catalyzed cyclization, the formation of cis-cyclohexanols is in remarkable contrast to Lewis acid-catalyzed cyclization to afford the trans products.