Medical diagnosis can be studied using various sources of information, such as medical and hospital discharge records and laboratory measurements. These sources do not always concur. The objective of the present study was to assess the sensitivity, specificity, and positive and negative predictive values of hospital discharge diagnosis compared with clinical laboratory data for the identification of hyponatremia. Patients with hyponatremia were selected from a hospital information system determined by the International Classification of Diseases, 9th edition (ICD-9). The validity parameters for hyponatremia (ICD code 276.1) were estimated by comparison with accurate serum sodium (Na+) levels. A total of 2632 cases of hyponatremia were identified using laboratory measurements (Na+ < or =135 mmol/L). The sensitivity of ICD coding for hyponatremia was maximally about 30% for patients with very severe hyponatremia (Na+ < or =115 mmol/L). Corresponding specificities were high (>99%). In 87% of the cases with severe hyponatremia (Na+ < or =125 mmol/L), other discharge ICD codes reflecting severe morbidity were found. This study suggests that ICD codes for hyponatremia represent only one third of the patients admitted to the hospital and experiencing hyponatremia. About two thirds of the patients with hyponatremia were classified as hospitalized for other reasons. To assess the validity of case finding of patients with hyponatremia, the use of analytical techniques, such as certain laboratory measurements, is advisable.