A cold strain index (CSI) based on core (T_core) and mean skin temperatures (_sk) and capable of indicating cold strain in real time and analyzing existing databases has been developed. This index rates cold strain on a universal scale of 0-10 and is as follows: CSI = 6.67(T_{core t}  T_{core 0}) · (35  T_{core 0})¹ + 3.33(_{sk t  sk 0}) · (20  _{sk 0})¹, where T_{core 0} and _{sk 0} are initial measurements and T_{core t} and _{sk t} are simultaneous measurements taken at any time t; when T_{core t} > T_{core 0}, then T_{core t}  T_{core 0} = 0. CSI was applied to three databases. The first database was obtained from nine men exposed to cold air (7°C, 40% relative humidity) for 120 min during euhydration and two hypohydration conditions achieved by exercise-heat stress-induced sweating or by ingestion of furosemide 12 h before cold exposure. The second database was from eight men exposed to cold air (10°C) immediately on completion of 61 days of strenuous outdoor military training, 48 h later, and after 109 days. The third database was from eight men repeatedly immersed in 20°C water three times in 1 day and during control immersions. CSI significantly differentiated (P < 0.01) between the trials and individually categorized the strain of the subject for two of these three databases. This index has the potential to be widely accepted and used universally.