Fluorescence energy transfer between donor and acceptor molecules in nonuniform molecular distributions is considered. In particular, we investigate the effect of the molecular distributions on the time-resolved fluorescence intensity decay of the donor and provide exact analytical expressions for the ensemble-averaged donor decays when the acceptor molecules are distributed uniformly within a slab of a finite thickness. The effect of the donor distribution on the energy transfer is also examined. Importantly, the derived formulas display both asymptotic limits for the two- and three-dimensional molecular distributions, and the crossover between them. Analytical results show very good agreement with numerical lattice model simulations. On this basis, we introduce new procedures for the energy transfer data analysis that provide a direct estimation of the slab thickness and the acceptor concentration from a single time-resolved fluorescence measurement. (C) 2003 American Institute of Physics.