The present study aimed to investigate the association between treatment outcome and the assimilation of problematic experiences during psychotherapy. The assimilation process is described in an eight-level sequence, the Assimilation of Problematic Experiences Scale (APES); insight is conceived as one level in this developmental sequence. This level is considered central as related to decreases in distress and symptomatic improvements. Based on treatment outcome measures, two contrasting groups were selected from a randomized controlled trial. The cases, three successful and three unsuccessful, included clients hospitalized for moderate or severe depression who received a short-term dynamic psychotherapy. Thirty-six session transcripts (six per client) were rated for assimilation level using the APES. In an exploratory purpose, linear mixed effects models were performed to compare successful and unsuccessful cases on their assimilation evolution. Results were partially consistent with the observations of previous studies linking a successful treatment outcome with insight events. Two successful cases reached the insight level, while none of the unsuccessful cases did. The quantitative analyses revealed that both groups presented a significant and similar increase in assimilation during psychotherapy. This study raised different issues concerning the association between assimilation, insight events, and treatment outcome.