We report a monosomy 18p syndrome in a male patient with polyglandular autoimmune syndrome (PAS) type IIIA. A 34-year-old mentally retarded diabetic male patient with short stature, wide earlaps, old-looking face, straight nasal bone, atrophic mouth, drooping cheeks, full teeth loss, and soft, weak and sparse white hair was admitted to the outpatient endocrinology clinic. Chromosome analysis of the patient revealed 46,XY,del(18)(p11.2). He was also diagnosed with autoimmune thyroiditis, primary hypothyroidism and diabetes mellitus type 1. We concluded that monosomy 18p syndrome may be associated with autoimmune diseases and if this is suspected, patients should be examined for an endocrine deficiency.