Objective: The present study examined the effects of sudden gains on treatment outcome in a randomized controlled trial including individual cognitive therapy (CT) and interpersonal therapy (IPT) for social anxiety disorder (SAD). Method: Participants were 67 individuals with SAD who received 16 treatment sessions. Symptom severity at each session was assessed using the Social Phobia Weekly Summary Scale (Clark et al., 2003). Results: Results indicate that 22.4% of participants experienced a sudden gain during treatment. Individuals with sudden gains had significantly lower social anxiety symptoms at post-treatment and follow-up compared to individuals without sudden gains. Sudden gains in CT and IPT had similar magnitudes, frequencies, and timings. However, sudden gains resulted in lower levels of post-treatment symptoms in CT compared to IPT. Cognitive changes did not precede sudden gains, but sudden gains resulted in cognitive changes. Conclusions: Sudden gains in CT and IPT for SAD are predictive of long-term outcome. In addition, the effect of sudden gains may be greater in CT compared to IPT. (Contains 1 table, 2 figures and 2 footnotes.)