Sudden gains are abrupt and substantial improvements in symptoms. This study used the Outcome Questionnaire-45 (OQ-45; Lambert et al., 1996) to characterize sudden gains occurring in a cognitive-behavioral therapy training clinic. Also, gradual gainers were identified and used as a comparison group. Sudden gains were identified in 23% of patients and in 29% of those who entered treatment in the clinical range on the OQ, within the range of prevalence established by previous sudden gain studies in the context of randomized controlled trials (RCT's). As in earlier research, sudden gains tended to occur early in therapy. However, sudden gains were more likely to be reversed than in RCT's. Gradual gains occurred for 54% of nonsudden gainers; they were of similar magnitude to sudden gains but occurred later in therapy. Sudden gainers showed significantly better outcomes at the end of treatment than did gradual gainers. Sudden gains have clear prognostic significance, but more research is needed to determine why they may be more transient in naturalistic studies or studies involving less experienced therapists than they are in RCT's. © 2010 Wiley Periodicals, Inc. J Clin Psychol 00:1–14, 2010.