Over a twelve-month period all 3918 stool samples sent to our institute were investigated for Blastocystis hominis. This protozoon was detected in 384 samples. The stools of 50 healthy controls were negative. In 56 positive cases detailed clinical information was obtained: 26 of these patients had diarrhea, while only extraintestinal symptoms had been recorded in 10 cases. Blood eosinophilia was observed in 8 patients. Of 16 specifically treated patients, 10 responded to therapy. Although the pathogenic significance of B. hominis is still unclear, we believe our observations justify antibiotic therapy in selected cases.