The purpose of the present study was to investigate the impact of subclinical psychological difficulties, as assessed by the Multilevel Classification System for Sport Psychology (MCS-SP; Gardner & Moore, 2004b, 2006), on the efficacy of the Mindfulness-Acceptance-Commitment (MAC; Gardner & Moore, 2004a, 2007) performance enhancement intervention. Thirteen collegiate field hockey and volleyball athletes participated in a 7-week MAC protocol, and their results were compared to those of a control group of 7 same-sport athletes. Nonparametric analysis of the data offers additional support for MAC as a strategy for enhancing the athletic performance of collegiate athletes and suggests the importance of the accurate assessment of subclinical psychological difficulties to ensure the successful application of sport psychology interventions. In essence, these results suggest that the presence or absence of subclinical psychological difficulties may serve as a moderating factor in performance enhancement efforts.