The molecular geometries of the most stable conformers of cis-cycloalkenes, CnH2n-2, with n = 6-8, were optimized by. both the MM4 molecular mechanics force field and by MP2 calculations using the 6-31G** basis set. Both computational methods agree that in each of these molecules the olefinic carbon atoms show small but definite pyramidality, contrary to the usual assumption made in structural studies. This is in agreement with both computational and experimental studies of related systems and can be understood in terms of the molecular mechanics model.