  We calculate the twist three distribution f_\perp(x,k^\perp) contributing to
Cahn effect in unpolarized semi-inclusive deep inelastic scattering. We use
light-front Hamiltonian technique and take the state to be a dressed quark at
one loop in perturbation theory. The 'genuine twist three' contribution comes
from the quark-gluon interaction part in the operator and is explicitly
calculated. f_\perp(x,k^\perp) is compared with f_1(x,k^\perp).
