We describe a boy, 10 years and 5 months old, who developed acute adrenal gland insufficiency which was confirmed by hormonal investigation. Abdominal magnetic resonance imaging showed unilateral enlargement of the right adrenal gland, whereas the other gland seemed normal – no cause was apparent. Three months later the patient presented with thrombosis in the right femoral vein and in the veins of the right leg. Autoantibodies against cardiolipin were strongly positive, while antinuclear antibodies and antibodies against double-stranded deoxyribonucleic acid were absent. There was no evidence of antiphospholipid syndrome associated with drugs, connective tissue disorders, or malignancies, strongly suggesting the diagnosis of primary antiphospholipid syndrome. The development of adrenal insufficiency has been reported in primary antiphospholipid syndrome due to adrenal hemorrhage following vascular occlusion of adrenal vessels or secondary to anticoagulant therapy. It was interesting to note that in our patient adrenal gland insufficiency preceded other clinical evidence of the syndrome by 3 months. The primary antiphospholipid syndrome should be considered a possible cause of Addison’s disease when the etiology is not obvious.