For diffusive many-particle systems such as the SSEP (symmetric simple exclusion process) or independent particles coupled with reservoirs at the boundaries, we analyze the density fluctuations conditioned on the current integrated over a large time. We determine the conditioned large deviation function of the density by a microscopic calculation. We then show that it can be expressed in terms of the solutions of Hamilton–Jacobi equations, which can be written for general diffusive systems using a fluctuating hydrodynamics description.