Previous medical and dental studies report wide variations in service rates across small areas, regions and providers. A major source of this variation appears to be the pattern of clinical decisionmaking of the provider. This argument was tested using dentist service rates (the average number of services provided per patient) calculated from 1984-1985 claims data for a population of well-educated, middle-class patients in Washington state, U.S.A. (N = 23,153). Service rates were calculated for each dental procedure in the following pairs of alternative treatments: crown vs amalgam or crown build-up; root canal therapy vs extraction; and fixed bridge vs removable partial denture. For each pair, dentists identified patient (e.g. cost, patient preference) and technical (e.g. periodontal status, tooth damage) factors which they considered to be important in choosing therapy. Regression analysis revealed that the technical factors explained little variation in the rates, while at least one patient factor was significant across services, except prosthetics. Environmental characteristics, structural features of the practice, and the dentists' practice beliefs also explained variation in the rates.