Objective: To examine the test-retest reliability of a new interviewer-based psychiatric diagnostic measure (the Preschool Age Psychiatric Assessment) for use with parents of preschoolers 2 to 5 years old. Method: A total of 1,073 parents of children attending a large pediatric clinic completed the Child Behavior Checklist 1 1/2-5. For 18 months, 193 parents of high scorers and 114 parents of low scorers were interviewed on two occasions on an average of 11 days apart. Results: Estimates of diagnostic reliability were very similar to those obtained from interviews with parents of older children and adults, with kappas ranging from 0.36 to 0.79. Test-retest intraclass correlations for DSM-IV syndrome scale scores ranged from 0.56 to 0.89. There were no significant differences in reliability by age, sex, or race (African American versus non-African American). Conclusions: The Preschool Age Psychiatric Assessment provides a reasonably reliable standardized measure of DSM-IV psychiatric symptoms and disorders in preschoolers for use in both research and clinical service evaluations of preschoolers as young as 2 years old. (Contains 1 figure and 5 tables.)