Occupants of buildings consciously interact with environmental systems to satisfy their specific needs for comfort. Field investigations have indicated that this adaptive behaviour of the occupants is a key factor in affecting the energy consumption of buildings and can change the energy use of similar buildings by a factor of 2. This study aims to reveal the factors in determining lighting energy consumption of open plan offices with the emphasis on the roles of occupant adaptive actions. Field investigations of open plan offices in Seoul, Korea were conducted from 20 July to 29 September. Internal and external illuminance and the use of lighting were monitored at ten minute intervals. Seventy seven office workers filled in the questionnaire survey forms five times a day for 40 days. Results reveal that there were close relationships between prevailing illuminance levels on the work plane and luminous comfort. This study presents the hourly profiles of both lighting use patterns and lighting energy consumption, and investigates the lighting energy consumption of open plan offices in relation to internal illuminance, visual comfort and lighting system use patterns. (c) 2011 Elsevier B.V. All rights reserved.