Diffusion-weighted signal from the brain, S, deviates from monoexponential dependence on the b-factor. This property is often referred to as biexponential diffusion, since the corresponding model fits data well. The aim of this study is to examine the necessity of using the biexponential model in homogeneous voxels under isotropic diffusion weighting up to b = 2.5 ms/microm(2). The model is compared to the cumulant expansion of ln S in a power series in b, which takes its origin in fundamental properties of the diffusion-weighted signal, but diverges at large b. The absence of statistically significant evidences for the biexponential diffusion is demonstrated in gray matter. The cumulant expansion terminated after the term b(2) describes data equally well with fewer adjustable parameters. The biexponential model is preferable in voxels with a partial volume of CSF.