Sex differences in tendon and ligament injury risk are reported: the relative risk of tearing the anterior cruciate ligament (ACL) is two to six times greater in young female compared to male athletes even after socioeconomic, health, and lifestyle background variables and the level of sports participation have been taken into account. In order to understand the effect of hormones on ACL injuries, it is necessary to consider the effect of testosterone, estrogen, and progesterone on connective tissue and muscles, in both men and women. Studies should, at the beginning, monitor periodical hormone excretion (per day/seasonal; at rest/under stress; in a state of satisfaction/disappointment; etc.) for each individual, for the purpose of determining the situations when sudden changes in hormone concentration levels occur that could affect ACL injury in different ways. This would especially contribute to the understanding of the effect of hormone concentrations in women, in whom the phase (or to be more precise the day) of the menstrual cycle additionally complicates the interpreting of the results.