Self-consistent theory of the early spinodal decomposition stage is developed. Calculations show the existence of two maxima in the wave number dependence of the structure factor S(q,t). The main maximum grows with time and has a position between q(saddle) (14) and q(m) (1) at a finite wave number q(+). The second one is localized near q similar or equal to 0, its height being practically independent of time, Characteristics of the main maximum are sensitive to the initial conditions. Experiments confirm these predictions.