The overtraining syndrome (OTS) is largely a diagnosis of exclusion. The aim of this study was to investigate the compact Profile of Mood State (POMS) questionnaire as an early and accurate indicator for the diagnosis of OTS and the reliability of such findings in a group of athletes diagnosed with OTS, in comparison with a non-OTS (NOTS) group. Athletes were from athletic clubs in Gauteng, South Africa. The study population included 10 endurance athletes who completed the compact version of the POMS test which is based on 65 questions using a 4-point adjective rating scale which measure 6 identifiable moods of affect states: tension-anxiety; depression-dejection; anger-hostility; vigour-activity; fatigue-inertia; and confusion-bewilderment. Anger, vigor and fatigue all reflected p values < 0.0001, thus showing reliability in predicting OTS. The study showed significant differences in mood states between the OTS and the NOTS group. Only tension was similar in both groups. The POMS test was found to be a promising tool in the early diagnosis of OTS.