Using linearised rate equations, the intensity fluctuations in the output of c.w. d.h. GaAlAs-diode lasers have been calculated in the low-frequency range, where the flicker noise dominates. The theoretical results are compared with the optical-intensity fluctuations, which are obtained experimentally. Our measurements indicate that 1/f noise in lasers is mainly caused by 1/f noise from carrier transport in the bulk material and the contacts.