An empirical formula for gamma-ray buildup factors in two-layer shields is proposed. The values of the parameters are given for the formula when fitted to the dose buildup factors, calculated by the invariant imbedding method, for normally incident gamma rays penetrating two-layer shields comprised of combinations of water, iron, or lead slabs. The results from the present formula are in excellent agreement with the basic data in the 0.66- to 10-MeV energy range and for total thicknesses up to ∼20 mfp. The parameters used in the formula change smoothly with the source energy. Therefore; the buildup factor for any arbitrary energy can easily be estimated by interpolation of the parameters with respect to energy. The buildup factors provided by this formula also are in good agreement with those from transport calculations for two-layer shields consisting of water and lead. These factors also agree with those measured by two ionization chambers for 60Co radiation penetrating two-layer shields comprised of water, iron, or lead slabs.