[en] A simple relativistic expression for the structure function of a two-body S-wave bound state in lambdaphi³ theory is obtained at all values of the scaling variable in terms of the Deser-Gilbert-Sudarshan-Ida spectral function of the Bethe-Salpeter wave function in the ladder approximation. A similar result is given for a fermion-antifermion bound state with approximate wave function derived from conformal invariance. At small x the structure function has the expected peak broadened by the binding effect and for a fermion-antifermion S-wave bound state it behaves like (1 - x)² as x → 1