A method using multiplex PCR followed by cycle-sequencing has been developed to detect mutations in the FIX gene. The procedure was evaluated in 45 severe or mild haemophilia B patients from 45 unrelated families. At least one deleterious mutation was identified in every haemophiliac demonstrating the efficiency of the method. Furthermore the described procedure offers many advantages compared to other screening detection methods: it is fast (less than 48 h), simple (partly automated) and of relatively low cost (it requires only one PCR).