Under strong nanosecond excitation, we demonstrate that II–VI semiconductor quantum dots exhibit optical gain up to room temperature. The CdS nanocrystallites, fabricated by sol-gel process, are embedded in a sodium borosilicate glass matrix. The gain is spectrally broad and the crossing between absorption and gain is well above the bulk absorption band edge energy, since our nanocrystallites are in the intermediate quantum confinement regime. The gain has a long tail extending to the low energy side of the absorption edge. This tail was shown theoretically to originate from the multiple excited states involved in the recombination between two and one electron-hole pairs.