Dehydration is the most common fluid and electrolyte disorder among the elderly, yet risk factors are not known. This study identifies risk factors for dehydration in acutely ill nursing home residents. All 339 elderly resident of two nursing homes who developed an acute illness requiring hospitalization during 1984 were included in the study. The 173 patients having a serum Na less than 150 mg/dL and blood urea nitrogen to creatinine ratio (BUN:Cre) less than 20 were designated controls; 91 patients having a serum Na greater than 150 mg/dL or a serum BUN:Cre greater than 25 were designated cases. Odds ratios (OR) and confidence intervals were calculated for age, sex, chronic conditions, acute illnesses, medications, functional status measures, and season. Acutely ill dehydrated patients were female (OR, 3.3); over 85 years old (OR, 2.2); had more than four chronic conditions (OR, 4.0); took more than four medications (OR, 2.8); and were bedridden (OR, 2.9). Among the most severely dehydrated (serum Na greater than 150 mg/dL and BUN:Cre greater than 25), the odds ratios for the above factors were strengthened and other factors, such as inability to feed oneself and type of acute diagnosis, emerged as risk factors. Among the variables unrelated to functional status, laxatives (OR, 3.2) and chronic infections (OR, 1.8) were risk factors. We conclude that a group at high risk for dehydration can be defined and that they are better characterized by the number of chronic diseases and debilitated functional status than by acute disease processes.