A systematic analysis of Hospital Episodes Statistics (HES) data was done to determine the effects of the 2017 WannaCry attack on the National Health Service (NHS) by identifying the missed appointments, deaths, and fiscal costs attributable to the ransomware attack. The main outcomes measured were: outpatient appointments cancelled, elective and emergency admissions to hospitals, Accident & Emergency (A&E) attendances, and deaths in A&E. Compared with the baseline, there was no significant difference in the total activity across all trusts during the week of the WannaCry attack. Trusts had 1% more emergency admissions and 1% fewer A&E attendances per day during the WannaCry week compared with baseline. Hospitals directly infected with the ransomware, however, had significantly fewer emergency and elective admissions: a decrease of about 6% in total admissions per infected hospital per day was observed, with 4% fewer emergency admissions and 9% fewer elective admissions. No difference in mortality was noted. The total economic value of the lower activity at the infected trusts during this time was £5.9m including £4m in lost inpatient admissions, £0.6m from lost A&E activity, and £1.3m from cancelled outpatient appointments. Among hospitals infected with WannaCry ransomware, there was a significant decrease in the number of attendances and admissions, which corresponded to £5.9m in lost hospital activity. There was no increase in mortality reported, though this is a crude measure of patient harm. Further work is needed to appreciate the impact of a a cyber attack or IT failure on care delivery and patient safety.