The linewidth of a room-temperature free-running 940-nm cw single-frequency 1-mm-long quantum-well InGaAs/AlGaAs diode laser in a 10-cm-long external cavity has been measured as a function of the output power by the heterodyne technique. The power-dependent component of the observed Lorentzian linewidth displays a linewidth –power product of 28 ± 4 k Hz mW, which is 2 orders of magnitude larger than that predicted from the spontaneous emission model. The shot-noise model of Welford and Mooradian [Appl. Phys. Lett. 40, 560 (1982)] for the power-independent linewidth has been extended to account for the observed power-dependent linewidth.