Background During the last years, the concept of frailty has become increasingly familiar in many surgical specialties, including cardiac surgery. One of the main characteristics of frailty is its potentially reversible nature. With the aim of reducing the incidence of complications after surgery, efforts are being made to preoperatively decrease the level of frailty, however, the effect that the surgery itself may have in the postoperative frailty status is still unknown. Methods A prospective cohort of 137 patients aged 70 or more undergoing cardiac surgery with the use of cardiopulmonary bypass were recruited at three university-affiliated hospitals in Spain. Frailty status was assessed preoperatively and six months after surgery using two different validated frailty measurements: The Fried frailty scale and the Clinical Frailty Scale. We analyzed the changes in these frailty scores and the transitions between frailty status after surgery. Results Prevalence of frailty was 27.5% with the Fried Frailty Scale and 11% with the Clinical Frailty Scale. Even though frail patients presented a higher incidence of postoperative major complications than their robust counterparts, up to 50% of frail patients improved their frailty status after surgery. The distribution of frailty groups at baseline and six months after surgery changed significantly, decreasing the overall burden of frailty after surgery. Besides, robust patients showed a slightly worsening of their previous frailty scores. Conclusions Frailty is a multifactorial and dynamic condition that can be significantly reduced after cardiac surgery, suggesting that cardiac pathology is an important contributor to the preoperative patient’s frailty status.