Background: Clinical guidelines should be updated to maintain their validity. Our aim was to estimate the length of time before recommendations become outdated. Methods:We used a retrospective cohort design and included recommendations from clinical guidelines developed in the Spanish National Health System clinical guideline program since 2008. We performed a descriptive analysis of references, recommendations and resources used, and a survival analysis of re commendations using the Kaplan-Meier method. Results:We included 113 recommendations from 4 clinical guidelines with a median of 4 years since the most recent search (range 3.9-4.4 yr). We retrieved 39 136 references (range 3343-14 787) using an exhaustive literature search, 668 of which were related to the recommendations in our sample. We identified 69 (10.3%) key references, corresponding to 25 (22.1%) recommendations that required updating. Ninety-two percent (95% confidence interval 86.9-97.0) of the recommendations were valid 1 year after their development. This probability decreased at 2 (85.7%), 3 (81.3%) and 4 years (77.8%). Interpretation:Recommendations quickly become outdated, with 1 out of 5 recommendations being out of date after 3 years. Waiting more than 3 years to review a guideline is potentially too long. Abstract