Leptogenesis is an attractive mechanism to generate the matter excess of the Universe. It has the tantalising feature that low energy CP violation among leptons might be related to the baryon asymmetry. However, this is not in general realised, although it can arise in some models. In this talk, I introduce a Jarlskog invariant for the CP violation of leptogenesis. The invariant can be evaluated in terms of left-handed neutrino parameters. The limits in which MNS phases are relevant to leptogenesis can easily be read from this formula.