Purpose: Studies suggest that women have worse treatment outcome than men after acute Achilles tendon rupture (ATR). The aim of this study was to investigate if sex and age affect treatment outcome after ATR. Methods: The study was performed as a registry study in the Danish Achilles tendon Database. The primary outcome was change in the Achilles tendon Total Rupture Score (ATRS) from baseline to 4 months, 1 year and 2 years follow-up. Variables of interest were sex and age group (< 40 years, 40–65 years and > 65 years). Results: Data were collected from April 2012 to March 2018. Five-hundred and sixteen patients (416 men, 100 women) were included in the study population. At baseline, women scored 4.3 points lower in ATRS compared to men. No statistically significant difference between the sexes regarding change in ATRS were found. Women scored statistically significantly less in absolute ATRS at 1 year follow-up (mean difference 9.4; 95% CI 3.8, 14.9; P = 0.03). Patients older than 65 years scored statistically significantly more in ATRS change compared to patients between 40–65 years (mean difference 12.8; 95% CI 6.1–19.5; P < 0.001). Conclusion: This study did not show a statistically significant or clinically relevant difference between the sexes in ATRS change from baseline to follow-up. The mean difference in ATRS change between patients older than 65 years and patients between 40–65 years was clinically relevant with better outcome for patients older than 65 years. When comparing ATRS between groups with an unequal sex distribution, the findings of a baseline difference and a difference in absolute ATRS at 1 year follow-up between the sexes, advocate for reporting of sex-specific data or for use of change in ATRS from baseline to follow-up instead of absolute ATRS. Level of evidence: Level III.