X-linked adrenoleucodystrophy (ALD) has been shown to be one of the most frequent causes of Addison's disease in men. It is characterized by an impaired peroxisomal beta-oxidation of very long chain fatty acids and is associated with mutations of the ALD gene resulting in a defective peroxisomal membrane transport protein. There is a striking variability of endocrinological and neurological symptoms in patients with ALD, with no clearly evident correlation between mutations of the ALD gene and the different neurological phenotypes. No data on endocrinological symptoms and the ALD genotype have been published so far. We report endocrinological, clinical, laboratory and molecular genetic data from 55 patients with ALD from 34 families. Endocrinological symptoms of adrenal insufficiency were observed in 33 patients, 20 of whom showed additional neurological symptoms of cerebral ALD or adrenomyeloneuropathy. Isolated neurological symptoms were seen in 12 patients; in nine patients there were neither endocrinological nor neurological symptoms. Mutations of the ALD gene (n = 28) were detected in 50 patients (including nine sets of brothers) from 32 families. No correlation was found between the ALD gene mutation and endocrinological dysfunction. However, we found that all sets of brothers were concordant for the endocrinological phenotype (cortisol synthesis was reduced in two sets and normal in seven sets), whereas four sets showed a discordant neurological phenotype. As yet unknown hereditary factors other than mutations within the ALD gene may interfere with the endocrinological phenotype more strongly than with the neurological phenotype of ALD.