Calculations of neutron streaming in gas-cooled fast reactors (GCFR) designed with fuel pins have not been made properly up to now. The usual approach for computing the diffusion coefficients fails for two reasons: (a) the voided region is located at the cell boundary, and (b) the pitch is such that two-dimensional infinite gaps extend through the reactor. For an infinite lattice, the diffusion coefficient will diverge, which means that, in principle, the diffusion theory is no longer valid. This fact has been more or less forgotten because most theories assume cylindrical cells and therefore remove this difficulty artificially. Introducing the real size of the reactor at the beginning, a new theory of the streaming, which generalizes the usual approach is developed; it appears as a buckling dependent term in the diffusion coefficient which diverges slowly for an infinite lattice. Fortunately, this term is small for usual reactor sizes, and one may, therefore, continue to use diffusion theory for practical calculations. The numerical applications to GCFR lattices show that the streaming was underestimated in the past.