Addison's disease may be difficult to diagnose because of the non-specificity of the most frequently occurring symptoms. Hyponatraemia, hyperkalaemia, and uraemia are commonly detected in sera from patients with Addison's disease. We report two patients in whom the diagnosis was delayed, despite typical biochemical abnormalities in their sera during initial presentation. A computer search of 4 weeks of biochemical data (9862 requests for electrolytes) indicated that these typical biochemical abnormalities occurred commonly in other patients, increasing the difficulty of the clinical biochemist in detecting Addison's disease. Awareness of this problem, in combination with adequate clinical information, should enable the clinical biochemist to assist in the earlier diagnosis of Addison's disease.