Background: This study investigated sudden reversals during cognitive therapy (CT), through the comparison of “in-session” activity and “out-of-session” life events. Method: The sample comprised 20 clients who experienced sudden gains during CT for depression: 10 who subsequently suffered a reversal of this gain and 10 matched clients who maintained progress. Measures of client resistance and therapist responses were scored by judges from session transcripts and clients completed measures of life events and depression at each session. This enabled the Reversal and Non-Reversal groups to be compared during pre-sudden gain, pre-reversal or matched sessions. Results: No differences were apparent between the Reversal and Non-reversal groups or between the pre-gain and pre-reversal sessions in terms of client or therapist “in-session” activities. In the Reversals group, 6 out of 10 clients recovered their level of symptom improvement, following the reversal. Therapists showed higher levels of reflection and self-disclosure with clients who did not recover their symptom gain following the Reversal than with clients who did recover their symptom gain following the Reversal. Conclusions: The study is discussed in terms of the manner by which reversals can be more effectively measured and researched.