The recent Patents Court decision in Edwards Lifesciences v Cook Biotech highlights the dangers of filing a patent application before ensuring that the interests of any co-inventors have been assigned. The proceedings concerned the validity and infringement of Cook's patent for an artificial heart valve. Mr Justice Kitchin found that the patent was invalid for obviousness over cited prior art and, in addition, had not been infringed by Edwards' device. During the proceedings an issue arose regarding the validity of the claimed priority date for the patent application. This was significant as it would determine which prior art was relevant and in particular, whether an article published close to the original filing date was available as prior art.