Hydrogen-bonded systems have a surprisingly simple equation of state in the mean field approximation. If we know the equation of state P-ref(T, V, N-1, N-2, ...) of the reference system (without hydrogen bonds), then the equation of state of the system with hydrogen bonds can be written as P(T, V, N-1, N-2, ...) = P-ref(T, V - Sigma (VijMij)-M-H, N-1, N-2, ...) - kT Sigma M-ij/V, where M-ij is the number of bonds of the corresponding kind and V-ij(H) is the volume change upon a bond formation. It is interesting that this equation is valid for fluids, gases, solids-as long as the system is far from the critical point and the basic presumptions of the mean field theory are valid. This equation as well as other useful thermodynamic relations for hydrogen-bonded systems are proved, and their consequences are discussed.