Though previous research has extensively reported that sudden gains are associated with superior treatment results, research on the long-term effects and stability of sudden gains is not as consistent. The current study explored the long-term stability of early sudden gains (ESGs) observed in a brief acceptance and values-based intervention for depression provided by novice therapists. The participants were 56 volunteers diagnosed with major depressive disorder. Among the participants, 23% experienced ESGs, i.e. they reached the status of improved or recovered in the Reliable Change Index (RCI; Jacobson & Truax, 1991) classification after only two sessions. The current study examined the level of depressive symptoms (BDI-II), psychological flexibility (AAQ-II), believability of depressive thoughts (ATQ-B), and hopefulness (ASHS) 6 and 12 months after the intervention. Of the original participants, 77% (n = 43) took part in the 12-month follow-up. The results showed that positive changes achieved during the two first sessions in the ESG group were maintained up to 12 months after the intervention. The ESG group remained improved or recovered in the RCI classification through the 12-month follow-up period. However, at the 12-month follow-up, there were only small differences between the ESG and the non-ESG groups. The results suggest that participants achieving ESGs show stable improvements lasting up to 12 months after the treatment.