In a serological survey among Dutch patients suspected of leptospirosis, using a recently developed enzyme-linked immunosorbent assay, a patient was traced with a high antibody titre to Hantaan virus. No anti-leptospira antibodies were detected in this 27-year-old man. Shortly before he had been admitted to the hospital with progressive dyspnoea and coughing, accompanied with high fever. An interstitial pneumonia was diagnosed. He subsequently developed a progressive renal failure with proteinuria and polyuria. Later a liver failure accompanied with thrombocytopenia, anaemia and coagulation disturbances occurred. Before an aetiological diagnosis was made, the patient was treated with erythromycin. The patient eventually recovered completely. Based on the clinical symptoms and the positive serology, it was concluded that the disease diagnosed had probably been caused by a Hantaan virus infection. The diagnostic value of Hantaan virus serology in patients with similar symptoms is stressed.