The present invention is based on the finding that administration of polypeptides comprising at least one Immunoglobulin single variable domains against vWF to human TTP patients provides a significant decrease in the time to response. The invention provides a polypeptide comprising at least one immunoglobulin single variable domain (ISVD) against von Willebrand Factor (vWF) for use in treating a vWF-related disease in a human in need thereof. The invention further relates to dosage unit forms, kits and medical uses for treating TTP.