Diagnosis and treatment of Strongyloides stercoralis infection can be difficult, and a high degree of clinical suspicion in patients who have visited an endemic area is required. We describe the epidemiology and clinical features of 152 prospectively identified cases of strongyloidiasis in an European region, and identify risk factors for the development of severe forms of the disease. This was a prospective study of all patients admitted to a single institution over an 8-year period. Patients (n=152) were mainly elderly male farmers (79%) who had acquired the disease by working barefoot in contact with soil and ingesting non-drinking water. Eosinophilia was a sensitive marker for the infection (82%). Twenty patients (13%) developed severe forms of the illness and six patients (4%) died. A significant association was found between severe forms of strongyloidiasis and steroid usage (OR 9.0, 95%CI 2.1-37.6), immunodebilitating illness (OR 10.1, 95%CI 3.2-32.3) and other immunosuppressive therapy (OR 13.7, 95%CI 2.9-58.7), but by logistic regression analysis, only immunodebilitating disease was as a risk factor (OR 2.1, 95%CI 1.78-2.43). S. stercolaris infection is endemic in the Spanish Mediterranean coast. The frequent development of severe forms of the disease, with a high mortality, makes early recognition and treatment essential.