Negative change occurring during psychological therapy is under-researched. There is currently no theory which unifies correlates identified by previous research. This study aimed to generate a theory of negative change by interviewing clients and therapists about their experience of therapy when reliable score deterioration on outcome measures had been observed. In-depth interviews were conducted with eight clients and four therapists about the process of therapy. Analysis was based on a positivist Grounded Theory methodology. The emerging model identified three major themes postulated to explain negative change as experienced by this sample. A context of adversity, the therapeutic process and help withdrawn, within the context of positive outcomes. Process issues incorporated categories around; feeling there was not enough helpful advice, talking about distressing issues, difference, the challenge of no change, relationship difficulties, ambivalence, losing hope and goals around getting support instead of change. The importance of paying attention to context and life events was highlighted. Many clients wanted further input and some did not think there had been a negative outcome, or noted that change was not instant. Varied process issues highlight the need to adjust interventions for clients and review outcome measures used.