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). METHODS 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.