Background: The Project on Research and Intervention in Monotonous work (PRIM) studied Danish workers for 3-4 years to determine the effects of monotonous work. The present study is a subset of that cohort and compares prevalence and incidence differences of carpal tunnel syndrome (CTS) between workers with highly repetitive work tasks and workers with varied work tasks. Methods: The baseline study included 731 participants. Follow-up examinations were performed after 6 and 18 months. The CTS diagnosis was based on symptom interviews and nerve conduction tests. The repetitiveness levels were determined with electrogoniometers and observation of cycle times. Results: The overall prevalence of CTS was 1.6% on the working hand and 0.7% on the other hand. There was a significantly increased risk of CTS for every 10-hr increase of repetitive non-forceful work (OR = 1.84; 95% CI = 1.06-3.19) on the working hand. This result, however, was based on few cases and a low prevalence in the control group. The overall annual CTS incidence was 0.62% on the working hand and 0.44% on the other hand. For the repetitive work tasks, the mean power frequencies ranged from 0.53 to 0.79Hz. Conclusions: In the baseline study, highly repetitive work was associated with CTS. The CTS incidence was too low to perform any analyses of exposure differences.