BACKGROUND: Worldwide, adults and children are at risk of adrenal insufficiency as a result of adrenal suppression from use of anti-inflammatory glucocorticoids and opiates, as well as infectious diseases. The adrenocorticotropin (ACTH) stimulation test is the reference standard for diagnosis of adrenal insufficiency but requires clinic attendance and venesection. Salivary cortisone reflects free serum cortisol, and samples can be collected at home and posted to a laboratory. We tested whether home waking salivary cortisone level could be used to screen for adrenal insufficiency. METHODS: A prospective, diagnostic accuracy study was performed in patients at high risk of adrenal insufficiency. Patients collected a home salivary sample on waking and then attended the clinical facility for an ACTH stimulation test. Salivary cortisone was measured by liquid chromatography–tandem mass spectrometry. Receiver-operating characteristic curves were computed, and positive and negative predictive values were calculated. RESULTS: Two hundred twenty patients were recruited. As measured by an ACTH stimulation test, the prevalence of adrenal insufficiency was 44%. The area under the receiver-operating characteristic curve for waking salivary cortisone as a predictor of adrenal insufficiency was 0.95 (95% confidence interval [CI], 0.92 to 0.97). Cutoffs to ensure a minimum of 95% sensitivity and specificity gave a negative predictive value of 96% (95% CI, 90 to 99) and a positive predictive value of 95% (95% CI, 87 to 99) to exclude and confirm adrenal insufficiency, respectively. Waking salivary cortisone data provided information similar to that of an ACTH stimulation test in 70% of participants. Eighty-three percent of patients preferred home salivary collection to clinic attendance. CONCLUSIONS: Home waking salivary cortisone sampling has accuracy for the diagnosis of adrenal insufficiency similar to that of a standard ACTH stimulation test. Patients found the at-home test to be more convenient than the hospital-based test. (Funded by the National Institute for Health Research.)