This paper examines a remarkable lawsuit in health care rationing. The Patients Association for Interstitial Cystitis sued the Dutch National Health Care Institute for alleged misconduct against Interstitial Cystitis patients, as the Institute decided that bladder instillations with chondroitin sulphate or hyaluronic acid are no longer covered by the basic health insurance. The patients' organisation challenged the Institute for basing its standpoint on scientific evidence; overruling clinical expertise and patients' experiences. While scientific advice is often solicited in public health issues, simultaneously, the authority of scientific advice is increasingly being questioned in the public domain. Also, the judiciary is frequently called upon to adjudicate in rationing decisions. Based on an ethnographic study of the National Health Care Institute, drawing on insights from the field of Science and Technology Studies, we analyse this lawsuit as a negotiation of what knowledge counts in reimbursement decisions.