Stress measurements with sub-monolayer sensitivity are performed to investigate the correlation between mechanical film stress and magneto-elastic anisotropy in epitaxial ferromagnetic monolayers. The magneto-elastic coupling B 1 of Fe(100) films is measured directly. Magnitude and sign of B 1 deviate from the respective bulk value. A strain-dependent correction of the magneto-elastic coupling coefficient B 1 describes the apparent thickness dependence of B 1 for film thicker than 10nm. For thinner films, the possible contribution of surface corrections is discussed to explain the almost constant B 1 . The implications of a modified magneto-elastic coupling for the anisotropy of ultrathin films is elucidated.