The aim of this research was to study headgear co-operation using an objective measuring instrument, the Compliance Science System (CSS). Forty-six patients were included in the investigation, 32 girls and 14 boys (10-15 years of age; mean 13 years). The patients, who were not informed that they were being monitored, were instructed to wear the headgear, with an electronic module timer attached to the neckstrap, for 16 hours per day. At the end of 2 months (T-1), the time for which the headgear was worn was measured. At this stage the patients were divided into two groups: group 1 (unco-operative patients) who wore the headgear for less than 16 hours per day, and group 2 (co-operative patients) who wore the headgear for at least 16 hours per day. Only the unco-operative patients were informed that they had been monitored, and a subsequent 4 month treatment period was initiated for both groups. The time was also measured at the end of the second (T-2) and third (T-3) 2 month treatment periods. The unco-operative patients increased their use of the headgear to approximately 4.5 to 6 hours per day (P < 0.05). All of the co-operative patients also used their headgear as recommended during the 4 month period.