This article, which is the first of a two-part series, presents results for the first objective of The American Dietetic Association (ADA) 1991 Dietetic Practice Study: to determine the characteristics of advanced-level dietetics practice. A nationwide mail survey of ADA members was conducted on a stratified random sample of 8,012 beyond-entry-level (registered before April 1988) registered dietitians who were members of dietetic practice groups (DPGs). The sample was supplemented with two randomly selected control groups of 1,000 entry-level and 1,000 beyond-entry-level registered dietitians. The overall response rate was 63.1%. The 5,852 usable returns were representative of the dietetics population surveyed. A model of advanced-level professional practice was developed that specified minimum necessary requirements for advanced practitioners on five components: education and experience, professional achievement, approach to practice, professional role positions, and professional role contacts; measurement of a sixth component, advanced-level practice performance, was unsuccessful. A series of validation analyses found the model to be a statistically sound and reliable means of distinguishing advanced practitioners from other groups of dietitians in 8 of every 10 cases. A total of 461 (8.9%) dietitians met all requirements of the model and were classified as advanced practitioners. Projected estimations of advanced practitioners in the population of beyond-entry-level ADA members who are also members of DPGs ranged between 2,126 and 2,640 dietitians (3.5% to 4.3% of the ADA membership).(ABSTRACT TRUNCATED AT 250 WORDS)