Severe combined immunodeficiency (SCID) of horses is an inherited autosomal recessive defect carried by pure Arabian horses and their crossbreds. SCID foals have a deficiency in the number and function of mature lymphocytes and are very prone to infectious diseases. The defect is caused by a mutation in the PRKDC gene coding for the catalytic subunit of the DNA activated protein kinase (DNA-PKcs). Identification of the causative mutation made it possible to design a genotyping test for identifying carriers of the defective allele and to use this information in selective breeding against SCID. Here, the first frequency sampling of the SCID allele in the Belgian population of Arabian horses is presented. Four hundred and thirteen animals were genotyped, 25 or 6.05% of which were found to carry the SCID allele. Calculations based on these results showed the SCID allele frequency to be around 3.1% and the occurrence of SCID to be 0.096%.