Background: The best choice of orthosis in the treatment of acute Achilles tendon rupture is still under debate. Objective: To investigate if choice of orthosis in the first 3 weeks of treatment affected patient reported outcome (Achilles tendon Total Rupture Score (ATRS)), tendon elongation (Achilles Tendon Resting Angle (ATRA) and Heel Rise Height (HRH)) and re-rupture. Methods: Registry study in the Danish Achilles tendon Database. Patients treated with cast and patients treated with walker in the first 3 weeks of treatment were compared using a linear mixed-effects model adjusted for potential confounders. Results: 1304 patients were included in the study. No clinically relevant difference was found: Adjusted mean difference (using walker the whole period as reference)(95% CI) ATRS after 1 year = 0.1(−3.0; 4.1), ATRS after 6 months = 2.0(−4.5; 5.8), ATRS after 2 years = 3.0(−0.7; 7.0), HRH difference = 0.6(−6.6; 8.2), ATRA difference = 0.03°(−1.5; 1.6), re-rupture(odds ratio) = 0.812(0.4; 1.61). Conclusion: Patients treated with cast the first 3 weeks after acute Achilles tendon rupture did not have better treatment outcome than patients treated with walker.