BACKGROUND. Despite the importance of hydration, limited research has been undertaken in Arabic populations. METHODS: Study 1: 88 adult military cadets and 32 school-based adolescents provided five sequential daily mid-morning urine samples. Hydration thresholds were produced using percentiles of estimated urine osmolality (Uosm) and urine color (Ucol). Study 2: 120 military cadets and 52 adolescents provided 1077 mid-morning urine samples and the Uosm:Ucol relationship was assessed using regression. Study 3: 21 adolescents participated in a four-week hydration campaign, providing urine samples pre (PreC), at the end of (EndC) and two weeks post (PostC) campaign. RESULTS: Study 1: Euhydration (41-60th percentile) was 881-970 mOsmol·kg-1 (adults) and 821-900 mOsmol·kg-1 (adolescents). Study 2: In both cohorts, Uosm and Ucol were associated (p<0.01): adults R2=0.33; adolescents R2=0.59. Study 3: Urine osmolality was significantly higher PreC compared to EndC and PostC. CONCLUSIONS: Urinary output of Arabic adolescents and military cadets was more concentrated than frequently recommended for euhydration. Further work in similar populations is required to determine if these values represent hypohydration or merely reflect dietary and cultural differences. In Arabic adolescents and adults, Ucol was an adequate indicator of hydration status. Favorable hydration changes were made following a school-based health campaign.