Difficulties in adherence to treatment for men who have sex with men living with HIV are linked to patterns of experiential avoidance that keep them away from self-care behaviors. According to this, this study aims to evaluate the effect of a brief protocol of two sessions of Acceptance and Commitment Therapy on psychological flexibility and adherence to treatment. A single case design AB + follow-up was performed with five men who have sex with men and are living with HIV. Changes were measured by visual analysis and non-overlapping measures for adherence to treatment behaviors; and by reliable change criterion for measures of psychological flexibility. After the application of the protocol, changes were observed in the behavior of adherence to treatment and in psychological flexibility in 4 of the 5 participants.