Objectives. Repeated exposure to cold is known to provoke adaptation, reflected as changes in thermoregulatory and endocrine responses. These adaptive mechanisms are relatively well known. However, the functional impact on performance has not been studied. Methods. The first study is a cross-sectional population study conducted in 1997 (n=2624) and 2002 (n=6825), in which Finns, aged from 25 to 74 years, were queried about their habitual cold exposure. The material from the questionnaire is combined with meteorological information. In the second study, we examined the type of cold acclimatisation, if any, that prevails in urban circumpolar residents. A controlled laboratory study was conducted where subjects were exposed to three 24-h periods of either warm, or cold, bright and dim light, during winter (n = 7) and summer (n = 8). Thermoregulatory responses, physical and cognitive performance, and circulating hormones were assessed. Finally, cold acclimation was studied by exposing ten subjects to cold, on ten successive days. Responses in thermoregulation, autonomic nervous system, physical and cognitive performance, mood and hormones, were followed. Results. New information is obtained about the magnitude of habitual cold exposure and how it is affected by age, gender, place of residence, as well as by the degree of cold exposure. In addition, knowledge of the prevailing pattern of cold adaptation of modern circumpolar residents and its effects on physicial performance, cognitive functioning and mood is received. The repeated cold exposure study reveals information on the successive stages of cold acclimatisation, as well as the interrelations between thermoregulation and performance. Significance. The medical community managing the health care of residents living in high latitude environments will have an improved understanding of the functional significance of cold adaptation, the specific environmental conditions and physiological mechanisms that affect performance and behaviour in these regions.(Int J Circumpolar Health 2003; 62(4):445-445)