Semiconductor ring lasers with a threshold current as low as 12.5 mA have been fabricated in GaAs/AlGaAs. The low threshold current is due to a passivating layer of silicon nitride and an optimised fabrication process that results in very smooth sidewalls. The threshold current was analysed and it was found that a considerable part of the current is lost by spreading towards the inside of the pillbox structure. The remaining loss mechanisms were identified as internal loss (1 dB), coupling loss (7–8 dB) and scattering loss (2–4 dB). The scattering loss was found to be independent of the radius for radii between 30 μm and 145 μm. The L–I curve displays several kinks, which are explained by shifts of the mode position and thus the coupling ratio at the Y-junction due to temperature changes with pumping.