Objective: To determine the incidence of hospital admissions and associated mortality rates for non-covid medical conditions during the covid-19 pandemic. Design: Nationwide, population based cohort study. Setting: Denmark from 13 March 2019 to 27 January 2021. Participants: All Danish residents >1 year of age. Main outcomes measures: Population based healthcare registries that encompass the entire Danish population were used to compare hospital admission and mortality rates during the covid-19 pandemic (from 11 March 2020 to 27 January 2021) with the prepandemic baseline data (from 13 March 2019 to 10 March 2020). Hospital admissions were categorised as covid-19 when patients were assigned a diagnosis code for covid-19 within five days of admission. All patients were followed until migration, death, or end of follow-up, whichever came first. Rate ratios for hospital admissions were computed using Poisson regression and were directly standardised using the Danish population on 1 January 2019 as reference. 30 day mortality rate ratios were examined by Cox regression, adjusted for age and sex, and covid-19 diagnosis was used as a competing risk. Results: 5 753 179 residents were identified during 567.8 million person weeks of observation, with 1 113 705 hospital admissions among 675 447 people. Compared with the prepandemic baseline period (mean hospital admission rate 204.1 per 100 000/week), the overall hospital admission rate for non-covid-19 conditions decreased to 142.8 per 100 000/week (rate ratio 0.70, 95% confidence interval 0.66 to 0.74) after the first national lockdown, followed by a gradual return to baseline levels until the second national lockdown when it decreased to 158.3 per 100 000/week (0.78, 0.73 to 0.82). This pattern was mirrored for most major diagnosis groups except for non-covid-19 respiratory diseases, nervous system diseases, cancer, heart failure, sepsis, and non-covid-19 respiratory infections, which remained lower throughout the study period. Overall 30 day mortality rates were higher during the first national lockdown (mortality rate ratio 1.28, 95% confidence interval 1.23 to 1.32) and the second national lockdown (1.20, 1.16 to 1.24), and these results were similar across most major diagnosis groups. For non-covid-19 respiratory diseases, cancer, pneumonia, and sepsis, the 30 day mortality rate ratios were also higher between lockdown periods. Conclusions: Hospital admissions for all major non-covid-19 disease groups decreased during national lockdowns compared with the prepandemic baseline period. Additionally, mortality rates were higher overall and for patients admitted to hospital with conditions such as respiratory diseases, cancer, pneumonia, and sepsis. Increased attention towards management of serious non-covid-19 medical conditions is warranted.