We have observed low-threshold lasing in a GaAs single quantum-well vertical microcavity. At 4 K the quantum-well emission linewidth is reduced, and the coupling efficiency β of the spontaneous emission to the lasing mode is improved. The laser threshold pump power obtained for a 200 Å GaAs quantum well pumped by a continuous wave Ti:sapphire laser was 5 mW, corresponding to 85 μW absorbed power. We have compared the experimental results with the theoretical prediction, and estimated the value of β to be 10−2.