Objective This longitudinal study tested the effectiveness of a group intervention designed to facilitate posttraumatic growth (PTG). Methods Sample consisted of 205 women diagnosed with non‐metastatic breast cancer who were either assigned to an intervention group (n = 58) or to a control group (n = 147). PTG, challenge to core beliefs, and rumination (intrusive and deliberate) were assessed at baseline (T1), at 6 months (T2), and at 12 months after baseline (T3). Results Results from the Latent Growth Modeling suggested that participants from the intervention group have higher levels of PTG. The challenge to core beliefs and the intrusive rumination have a moderator role on PTG, since group intervention is also linked to the enhancement of both variables. Conclusions Participation in the intervention group increase PTG. Challenge to core beliefs and intrusive rumination are improved by group intervention, which to a certain extent facilitate PTG.