Mendelian randomization (MR) can be variously dated as 67,1 33,2 283 or 164 (amongst others) years old. It is clear that in recent times there has been an exponential increase in publications on MR, both theoretical and applied.5,6 In addition to papers focused on MR, MR analyses are increasingly seen in the set of follow-up analyses for a genome-wide association study (GWAS), together with the obligatory bioinformatic and functional evidence. The Internationl Journal of Epidemiology compiled a special issue on MR to accompany the second international MR conference in 2015.7 The present issue repeats this for the fourth international conference. The speed of progress in the field is reflected in both the content of the issues, and by the fact that the volume of material now exceeds the capacity of a single issue. Further compilations of MR papers will appear in subsequent issues.