The authors describe the use of diagnostic supervision to identify common errors made by trainees in the application of DSM-lII to multiaxial evaluations in an outpatient clinic. Errors on all five axes were due primarily to misapplications ofdiagnostic criteria and conventions. Errors on axes I, IV, and V were most frequent. Axis I errors were cornrnonly due to confusion about the relationship of dysthyrnic disorder to major depression, neglect of substance use disorder diagnoses, and misuse of the adjustment disorder and V-code categories. On axis JV, the severity of psychosocial stressors was frequently overrated, based on severa! misconceptions. Axis V ratings were often erroneously over estimated because they were individualized rather than made on a uniform scale. No differences were found in the error rates of psychiatric residents compared with psychology interns except on axis 1, where interns made more errors. The authors discuss the implications of these errors for training residents and psychology interns in the use of DSM-llI.