The Norwegian Surveillance System for Communicable Diseases (MSIS) is based on reporting of cases of 72 mandatory notifiable diseases by clinical microbiological laboratories and diagnosing physicians. The aim of our study was to investigate a potential temporal association between measures against COVID-19 in February-April 2020 and incidence of other infectious diseases reported to MSIS.We compared the number of disease cases reported to MSIS during weeks 6-14 in 2020 with the median of cases reported in corresponding weeks during three previous years (2017-2019).Compared to the median of cases reported during corresponding weeks in three previous years, physicians and laboratories reported 47 % fewer cases (159 vs. 301) in week 12, 50 % fewer cases (131 vs. 261) in week 13, and 69 % fewer cases (77 vs. 252) in week 14. There was a reduction in the number of notifications of all included disease groups.The observed decline in reporting of diseases other than COVID-19 may indicate a reduced risk of communicable diseases due to comprehensive advice and the requirement for social distancing. However, it is also possible that the sensitivity of the surveillance system was affected by increased resource use on COVID-19 cases management.