We apply scanning voltage microscopy to actively biased multiquantum-well ridge-waveguide semiconductor lasers. We localize the source of a major and hitherto unexplained sample-to-sample difference in current-voltage characteristics to the responsible junction. This is found to correspond to the regrowth interface, subsequently confirmed through secondary ion mass spectrometry to have different doping profiles in the two cases. By comparing the internal voltage profile of the operating lasers, we found that a voltage difference of 0.44 V occurred within ∼100 nm of the regrowth interface in these laser structures, accounting for 88% of the difference in the measured series resistance. Additionally, 75% of the total device series resistance is associated with the structure's heterobarriers. These results relate nanoscopic measurements to macroscopic performance and are of significance in improving device understanding, design, and reliability.