Oxidative stress plays probably an important role in the etiopathogenesis of amyotrophic lateral sclerosis (ALS). It is known that bilirubin (BR) is an endogenous antioxidant. The aim of this study was to evaluate serum BR concentration in ALS patients. BR was determined by automated analyzer in the serum from 30 ALS and 26 healthy control group people. The study showed that serum BR concentration is significantly decreased in ALS patients with a long duration of ALS compared with patients with a short duration (P<0.05), and it is also significantly decreased in ALS patients with a moderate clinical state compared with control group patients (P<0.05). There was no significant difference in BR concentration between the groups of patients classified according to their age and sex, the clinical state, and the type of ALS onset (P>0.05). Results suggest a possibility of endogenous antioxidant system dysfunction in later phase of ALS. A decrease in BR concentration might diminish its protective effect against oxidative injury and could accelerate motor neuron degeneration.