In this paper we report some theoretical and experimental results concerning the realization of the phase-matching conditions for the internal second-harmonic generation in InGaAs quantum-well laser diodes emitting around 980 nm in order to enhance the conversion efficiency. The role of the phase-mismatching in the spectral distribution of the internal second-harmonic generation in the CW operation of the above mentioned lasers is also analyzed. The emission of pairs of narrow blue-green peaks having perfectly symmetrical spectral positions with respect to the central peak of pure second-harmonic generation at ∼ 480 nm is most probably enhanced by a mechanism of reciprocal cancellation of the respective phase-mismatch vectors. The theoretical results obtained by numerical integration of the coupled amplitude equations corresponding to the fundamental and second harmonic concerning the interaction length and generation efficiency are in good agreement with the experimental data. The obtained results are important for the assessment of the relationship between the structural parameters of the laser and the conditions which contribute to the stimulation of second-order optical nonlinearities in the laser active region.