We detected heavy infections of Blastocystis hominis in four individuals with intestinal obstruction due to cancerous growths. After surgery, the infections spontaneously resolved, without specific chemotherapy. It appears that the B. hominis infection was coincidental and not related to the neoplastic growth. We suggest that intestinal obstruction and concomitant stool retention, plus hemorrhage from cancerous lesions, may have permitted the more abundant growth of B. hominis. This is the first report of a possible relationship between intestinal obstruction and a concomitant B. hominis infection.