The controversy concerning the patentability of gene based diagnostics has somewhat abated since the decisions at the European Patent Office on the Myriad patents. Nonetheless, whether it is in the public interest that patents over gene based diagnostics that otherwise fulfil substantive criteria should be available, remains a thorny issue. This article focuses on the patentability of preimplantation genetic diagnosis (PGD), which provides a means to test embryos generated by in vitro fertilisation (IVF) for a variety of inherited diseases, allowing the doctor to select only those without the defect for implantation in the uterus. The public health policy arguments advanced against the Myriad patents and surrounding patents on genetic diagnostics generally could also apply to any patent application for a method which includes claims to PGD. This paper considers whether patent claims to PGD methods fall within the ambit of either the section 4(2) Patents Act 1977 'medical exclusion' (implementing the equivalent exclusion in Article 52(4) of the European Patent Convention), or the exclusion in Article 6 of Directive 98/44/EC on the Legal Protection of Biotechnological Inventions. © 2007 A B Academic Publishers.