OBJECTIVE To compare treatment of heavy and low users of dental services among adults in the Public Dental Service (PDS) in one of the biggest cities in Finland and to identify reasons for heavy use and to suggest improvements to care provision. METHODS All adults who attended the PDS in Espoo (pop. 227,500) in 2004 were allocated to a group (n = 3,173) who had made six or more dental visits and a comparison group (n = 22,820) who had three or fewer dental visits. The data were obtained from the patient register of the PDS. A sample of 320 patients was randomly selected from each group. Information on age, gender, number and types of visits, oral health status, treatment provided and fees paid was collected from treatment records. RESULTS 10.5% of the adults were found to be heavy users and their treatment made up 31.6% of all adult dental visits. The proportion of men was greater among heavy users and the heavy users were on average 6.6 years older than the low users. The mean total treatment time for heavy users was 5.5 hours and 2.0 hours for low users. Heavy users had more untreated and treated caries and more periodontal pockets than low users. Restorative, endodontic and prosthetic treatment needs characterised the heavy user group, while the low users most often received restorative and periodontal treatment only. CONCLUSIONS Our study indicates that complicated treatment needs of heavy users and lack of experience among the caregivers in dealing with them resulted in high numbers of dental visits for individual patients. The PDS should offer appropriate continuing education for its oral health care teams and organize a referral system offering specialist care for difficult endodontic, periodontal and prosthetic treatments.