The energy savings achieved using continuous dimming and automatic on/off photo-control systems for the lighting in large atrium spaces in two Canadian buildings were compared. Horizontal illuminance at various locations, the electric lighting system time-of-use and power consumption were measured in mid-summer and mid-winter. Extrapolation of these results indicated that the continuous dimming system gave an annual saving of 46% in lighting electrical consumption, whilst the automatic on/off system saved 11-17%. It was also observed that reductions in energy efficiency of 30-65% were caused by reduced dimming linearity and incorrect adjustment of the phases of the dimming control system, incorrect location of the photocell controlling the on/off system, improper maintenance of the skylight during winter (snow clearing), and over-sizing of the lighting system. 13 refs.