Aim To compare the number of appendicitis cases and its complications, during the first months of the COVID-19 pandemic in Sweden and the UK and the corresponding time period in 2019. Method Reports of emergency abdominopelvic CT performed at 56 Swedish hospitals and 38 British hospitals between April and July 2020 and a corresponding control cohort from 2019 were reviewed. Two radiologists and two surgeons blinded to the date of cohorts analyzed all reports for diagnosis of appendicitis, perforation, and abscess. A random selection of cases was chosen for the measurement of inter-rater agreement. Result Both in Sweden (6111) and the UK (5591) fewer, abdominopelvic CT scans were done in 2020 compared to 2019 (6433 and 7223, respectively); p < 0.001. In the UK, the number of appendicitis was 36% lower in April-June 2020 compared to 2019 but not in Sweden. Among the appendicitis cases, there was a higher number of perforations and abscesses in 2020, in Sweden. In the UK, the number of perforations and abscesses were initially lower (April-June 2020) but increased in July 2020. There was a substantial inter-rater agreement for the diagnosis of perforations and abscess formations (K = 0.64 and 0.77). Conclusion In Sweden, the number of appendicitis was not different between 2019 and 2020; however, there was an increase of complications. In the UK, there was a significant decrease of cases in 2020. The prevalence of complications was lower initially but increased in July. These findings suggest variability in delay in diagnosis of appendicitis depending on the country and time frame studied.