Background: Despite repeated calls for research on the efficacy and effectiveness of psychotherapy with people with intellectual disabilities there has been little progress in this area. This paper reports a naturalistic study of the effectiveness of individual psychodynamic psychotherapy provided in routine clinical practice. Method: The study was an open trial design with a 3-month follow-up period. Widely available and used psychotherapy outcome measures were adopted, but administered in an assisted completion format at intake, outcome and follow-up. Data were analyzed using repeated measures ANOVA. Effect sizes were also computed. Results: The results show statistically significant reductions in recipients' levels of psychological distress and interpersonal problems and an increase in self-esteem. Effect sizes were modest to large. Conclusions: The study shows that adults with intellectual disabilities can benefit from individual psychotherapy provided in routine clinical practice. The method has obvious limitations because of its uncontrolled, open trial design. However, the study had a recipient group that represents the client population and thus has greater generalisability to clinical practice. This study also provides some suggestions for routine audit and evaluation exercises and more rigorous designs.