OBJECTIVES: X-linked adrenoleukodystrophy(X-ALD) is a rare X-linked recessive metabolic disorder. The mutations in the ATP Binding Cassette Subfamily D Member 1 (ABCD1) gene account for the underlying molecular mechanism. Herein, we present two siblings with X-ALD due to a missense, presumably identical, ABCD1 mutation, who had extremely distinct clinical phenotypes. MATERIAL AND METHODS: Patient 1 (6y/o) was admitted with primary adrenal insufficiency (PAI). His VLCFA analysis and cranial MRI suggested the diagnosis of X-ALD with no cranial involvement. Although the PAI was successfully managed using hydrocortisone replacement therapy, during follow-up he was admitted with the complaints of perception impairment, seizures, loss of vision and deafness suggesting cranial involvement which was not able to be recovered despite intensive supportive therapies; in the end patient died. Patient 2 (21y/o) had mild symptoms of PAI with no organ manifestation. He was undertaken to a molecular genetics analysis for ABCD1 gene due to history of his brother. His VLCFA analysis revealed mildly elevated C26, C22 and C26/C22 ratio suggesting ALD diagnosis. However, his cranial imaging and other results were within normal limits. CONCLUSION: Two siblings with X-ALD due to presumably an identical, missense ABCD1 mutation and distinct clinical phenotype have confirmed the lack of phenotype-genotype correlation and proved the essential role of molecular genetics analysis in the early diagnosis. It is crucial to follow up for the development of cranial involvement and decide a bone marrow transplantation which is the only option that can prevent the progression of the disease, thus extend the lifespan.