The two main causes of primary adrenal disease are tuberculosis and auto-immune adrenal destruction. The latter is responsible for about 70% of the cases of primary adrenal insufficiency (Addison disease). Commonly referred to as a rare cause of adrenal failure is X-linked adrenoleukodystrophy (ALD), a demyelinating peroxisomal disorder affecting 1: 20,000 Caucasian males. Albeit primary adrenal insufficiency is a rare entity per se, we decided to study patients with idiopathic Addison disease and establish the frequency of ALD as a cause of adrenal insufficiency. The biochemical defect of ALD was found in 5 out of 24 patients. The small number of cases in our series led us to include in our analysis the published results of two other groups of investigators. This analysis indicates that the proportion of cases in which Addison disease is attributable to ALD is age dependent. It is highest when the adrenal insufficiency manifests before 15 years. This study clearly demonstrates that the proportion of ALD in patients presenting primary adrenal insufficiency has been under-estimated. CONCLUSIONS Addison disease manifesting during the first decade of life has a high likelihood of being the first sign of X-linked adrenoleukodystrophy.