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(1 0 0) 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 10 nm. 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.