To enable optimal care, critically ill children on intensive care units (ICU)s may benefit from rapid genetic  diagnosis. We developed a workflow incorporating whole genome sequencing and causal variant identification in as little as 4 days. We investigated 24 individuals with suspected rare diseases on ICU at Great Ormond Street Hospital using this workflow. We made a diagnosis in 10 individuals (42%). In 3/10 individuals (30%) the diagnosis significantly altered their clinical management.  Our workflow adheres to the standards for National Health Service diagnostic laboratories and we provide full details of our methods for others to use locally. (By Dr. Hywel Williams, https://jmg.bmj.com/content/early/2018/07/24/jmedgenet-2018-105396?papetoc )