A bilinear fault detection filter (BFDF) is proposed for bilinear systems with unknown inputs. Sufficient conditions are given for the existence of a BFDF and a strict BFDF for which the latter is sensitive to all faults. A particular choice of observer structure is shown to give explicit solutions both for the observer gain matrix and for establishing fault sensitivity. Structured actuator and component faults give rise to residuals with known directions enabling these faults to be easily isolated. A numerical example is used to demonstrate the BFDF design.