Although intestinal parasites are a possible cause of skin disorders, there are few case reports concerning the role of Blastocystis hominis in urticaria. To clarify this association, we determined the frequency of B. hominis genetic subtype in urticarial patients by stool culture and polymerase chain reaction (PCR) and evaluated the clinical and parasitological recovery of urticarial patients after treatment with metronidazole. Of 54 urticarial patients (group I), 18 (33.3%) were diagnosed as acute urticaria (group IA) and 36 (66.7%) were diagnosed as chronic (group IB). Thirty-three (61.1%) out of 54 urticarial (group I) patients were Blastocystis positive by stool culture and PCR. Out of these 33 patients, 21 were symptomatic and 12 were asymptomatic. The amoeboid form was found in 20 (95.2%) out of 21 symptomatic Blastocystis urticarial patients assuring their pathogenic potential. Of 50 normal control group (group II), four (8%) Blastocystis isolates were found with no amoeboid form. B. hominis subtype 3 was the only detected genotype in both groups. Of 20 symptomatic Blastocystis urticarial patients, 12 (60%) patients recovered symptomatically and parasitologically after one course of metronidazole. Recovery reached 100% on repeating the treatment for a second course with disappearance of the amoeboid form. It was concluded that acute urticaria of unknown etiology and chronic idiopathic urticaria patients who are resistant to the ordinary regimen of urticaria treatment might be examined for infection with B. hominis, in order to prescribe the proper specific anti-protozoan treatment.