AIM: Difficulties in social functioning have been observed in youth at clinical high-risk (CHR) of psychosis even in those who do not go on to develop a psychotic illness. Few treatment studies have attempted to improve social functioning in this population. The aim of this study was to conduct a randomized trial comparing the effects of Cognitive-Behavioral Social Skills Training (CBSST) with a supportive therapy (ST). METHODS: Both CBSST and ST were weekly group therapies, delivered over 18 weeks. This was a 2-arm trial with single-blinded ratings and intention-to-treat analyses. Assessments occurred at baseline, end-of-treatment, and 12 months after the baseline assessment. The primary outcome was social and role functioning and defeatist performance attitudes were the secondary outcome. Attenuated positive and negative symptoms, anxiety, depression, self-efficacy, and beliefs about self and others were examined as exploratory outcomes. RESULTS: There were no significant differences between the 2 groups at baseline or either of the 2 follow-ups. However, at follow-ups, in each group there were significant improvements in clinical symptoms. These could not be attributed to group treatment since there was no control or wait-list group. CONCLUSIONS: Since poor social functioning is one of the most observed difficulties in CHR individuals, and a decline in social functioning may be a significant predictor of later transition to psychosis, future work will be needed to find effective treatments for this decline in functioning for CHR youth.