A quantum theory of second-harmonic generation is presented in which use is made of the formal equivalence that exists between, on the one hand, the emission of a system ofNtwo-level atoms that interact via the electromagnetic field and, on the other, the generation of second harmonics. TheN-atom system is described in terms of the Dicke model, but in the so-called ‘restricted rotating-wave approximation’. An analytical, closed-form expression is found for the number of second-harmonic photons as a function of time. Also, a comparison with the semiclassical theory and with experiments is given.