Several recent investigations have demonstrated that Cognitive Therapy (CT) and Behavioral Activation (BA) are both efficacious treatments for depression (Butler, Chapman, Forman, & Beck, 2006; Dimidjian et al., 2006; Dobson, 1989; Gloaguen, Cottraux, Cucherat, & Blackburn, 1998; Hollon, Thase, & Markowitz, 2002; Jacobson et al., 1996). This investigation focuses on the treatment of Mr. X, a 62-year-old man with chronic treatment-resistant depression and comorbid personality pathology. After a course of 21 sessions of CT, treatment was switched to BA. We present daily mood data from 147 days of each treatment. Generalized least squares analyses of these time series data demonstrated that BA was associated with improved mood and these results were supported by clinical observation of improved functioning during the course of BA. We hypothesize that efforts at cognitive restructuring had an iatrogenic effect on this client and paradoxically triggered depressive rumination; these failed efforts provided further evidence in support of his underlying defectiveness schema. We also speculate that this client's dependent personality ran counter to CT's emphasis on autonomous homework assignments. Future randomized clinical trials are needed to investigate if nonresponders to CT benefit by switching to BA. (Contains 3 figures.)