This study was designed to determine serum osmolality and to investigate the association among serum osmolality, health-related factors and biochemical indices. Two hundred thirty seven elderly(86 male, 151 female : mean age 73.8) residing in the chungbuk area participated. Sociodemographic data and self-perceived health status were obtained by interview, and biochemical parameters were measured. The mean serum osmolaity was 301.2+/-10.1 mOsm/ kg(range 240.8~328.9 mOsm/ kg) and serum osmolality of the females(302.4 mOsm/ kg) was significantly(p<0.05) higher than that of the males(299.0 mOsm/ kg). Only 11.0% of the elderly were within the accepted normal range(280~295 mOsm/ kg) and the majority of the subjects were in a hyperosmolar state(60.8% : 296~307 mOsm/ kg). It was observed that serum osmolality was reduced with increasing age but not statistically significant. There was no significant difference in serum osmolality according to marital status. Serum osmolality was significantly lower(p<0.05) in the elderly with higher self-perceived health scores than the elderly with lower self-perceived health scores, however no significant difference was observed between the presence or absence of disease. Serum osmolality tended to be higher(p=0.06) in the elderly with difficulty in mobility than the elderly who have no problem in mobility. Serum osmolality tended to be higher(p=0.06) among drug users compared to that of non-users. There was a significant association between serum osmolality and serum albumin in females but not in males. Significant associations were shown between serum osmolality and serum transferrin in both sexes. However, there was no significant correlation between serum osmolality and hemoglobin or hematocrit except with hematocrit in females only. The results of this study indicated that the elderly were dehydrated and hydration state seems to influence health status and, the elderly should be encouraged to drink an adequate amount of fluids. Also, the hydration state is required to be assessed and considered for the interpretation of biochemical parameters.