We study the Lorenz-Haken instability in a laser with arbitrary mirrors reflectivity R. In the limit of slow population difference we demonstrate that in order to observe the instability the reflectivity must exceed the critical value R-c similar or equal to 0.5379, which coincides with that found recently for the multimode (Risken-Nummedal-Graham-Haken) instability. Several other differences with respect to the uniform field limit R --> 1 are presented. (C) 2000 Elsevier Science B.V. All rights reserved.