Pythiosis is a cosmopolitan granulomatous disease caused by an aquatic fungus Pythium insidiosum which usually occurs in horses, cattle, dogs, cats or fishes. There have been 28 cases of human pythiosis published in the literature. Twenty three patients have been reported from all over Thailand. Human pythiosis presents in one of three clinical forms: cutaneous or subcutaneous, systemic or vascular and ophthalmic (e.g., corneal ulcer or keratitis). Systemic antibiotics or antimycotics are not effective in the treatment of this infection. A saturated solution of KI gives a beneficial result only in the subcutaneous form. Surgical removal of the source of infection is the method of therapy of vascular and ophthalmic forms.