OBJECTIVE: To investigate the proportion of running-related injuries that can be classified as (i) being based on overload with an acute mechanism with a sudden onset, (ii) being overuse in the sense they are based on a repetitive mechanism with a sudden onset; and (iii) being overuse in the sense that runners gradually develop problems that lead to an injury. DESIGN: Cohort study with an 18-month follow-up. METHODS: English-speaking runners aged at least 18 years who were tracking running activities were included. Running-related injury during follow-up categorized into three groups: (i) those with overload injury; (ii) those with a running-related problem 7 to 28 days prior to reporting a running-related injury and (iii) those sustaining a running-related injury without any preceding problems. RESULTS: Overload injury accounted for 467 of 1,666 injuries (28%), while 1,199 injuries were overuse (72%). Among overuse injuries, the proportion of runners having a problem prior to injury occurrence was 6.9% [95%CI: 5.5%; 8.4%] 7 days prior to injury and 11.1% [95% CI: 9.4%; 13.0%] 28 days prior to injury. CONCLUSION: Most of the overuse injuries occurring in adult runners may be classified as sudden repetitive overuse injuries rather than injuries that are based on a gradual onset.