Correlations between degradation and device characteristics are investigated in InGaAsP/InP buried heterostructure lasers. The logarithm of threshold‐current increase rates is confirmed to be proportional to the voltage decrease at low current when injected carrier lifetime decreases during aging. When leakage current increases during aging, devices having a low initial voltage also tend to degrade to a large extent. These correlations hold within an operating range limited by the device structure and the material used during fabrication.