To assess the practicality of the recent Scientific and Standardization committee (SSC) of the International Society on Thrombosis and Haemostasis (ISTH) recommendations in respect of the classification of hemophilia we distributed samples from three untreated subjects with hemophilia A to 91 UK hemophilia centers (HCs), comprising 20 comprehensive care centers (CCCs) and 71 HCs. Laboratories were requested to perform their routine factor (F)VIII:C assays and to classify the severity of hemophilia. Median values of < 1 U dL-1 were obtained on two samples. However, for each of the two, approximately 30% of laboratories obtained results in the range 1-29 U dL-1 and 1-33 U dL-1 respectively. For one of these samples 17 laboratories diagnosed severe hemophilia despite obtaining FVIII:C levels in the range 1-5 U dL-1. The median FVIII:C for the third sample was 5.8 U dL-1 with a range of 1.5-36 U dL-1. For this sample eight centers diagnosed severe hemophilia. Fifty-four laboratories obtained a result > 5 U dL-1; 21 of these diagnosed mild hemophilia, 31 moderate hemophilia and two severe hemophilia. Results from CCCs were more accurate and more precise than those from HCs. Our results indicate a need for improved standardization of FVIII assays. In the UK there remains a lack of consensus in respect of the laboratory diagnostic criteria for the classification of hemophilia A.