Controlled spontaneous lifetimes are demonstrated in InGaAlAs/GaAs quantum dots confined in planar microcavities. Due to their independent and spectrally sharp light emission, the quantum dot emitters provide an excellent means for studying the spontaneous lifetime dependence on microcavity tuning, while maintaining experimental parameters such as temperature and pump intensity constant. The measured lifetime changes are compared with calculated results and show good agreement. ©1998 American Institute of Physics.