A formula is given for the natural linewidth of high-gain lasers, which is applicable to arbitrary three-dimensional geometries. This formula agrees with Petermann's result for lasers with transversely inhomogeneous gains (K-factor) and with previous results for the effect of small mirror reflectivities. It does not agree with the Schawlow-Townes (ST) formula used by most authors in evaluating the α=Δnr/Δni factor of conventional semiconductor lasers. The difference between the two formulas is significant when the mirror power reflectivity is less than about 0.6. Furthermore, the modified formula gives directly the linewidth of lasers coupled to long external cavities. Saturation effects, however, are neglected in this letter.