The emergence of new systemic treatments for alopecia areata underscores the importance of estimating rates of potential treatment safety events among individuals with alopecia areata. In this population-based cohort study, data linked across Danish population-based registries were used to examine the rates of the following safety events of interest before approval of Janus kinase inhibitor treatments by the European Medicines Agency: serious infections, herpes zoster infections, malignancies, arterial and venous cardiovascular events, and all-cause death in an alopecia areata cohort, defined as individuals ≥ 12 years old with hospital-treated alopecia areata, including its sub-types alopecia totalis and alopecia universalis. Incidence rates of the safety events of interest were computed and their associations with alopecia areata were estimated as standardized incidence ratios computed with regard to the age- and sex-matched general Danish population. The alopecia areata cohort included 2,778 individuals (472 with alopecia totalis/alopecia universalis) with a first-recorded diagnosis of alopecia areata in 1995-2016, followed through to the end of 2016, during a hospital admission or treatment at an outpatient clinic. Hospital-treated alopecia areata was associated with serious infections (standardized incidence ratio [95% confidence interval], 1.89 [1.72-2.06]), herpes zoster infections (1.83 [1.63-2.05]), lymphoma (3.44 [1.88-5.77]), arterial and venous cardiovascular outcomes (1.41 [1.12-1.75]), and death (1.16 [1.00-1.34]).