BACKGROUND: Updating is important to ensure clinical guideline (CG) recommendations remain valid. However, little research has been undertaken in this field. We assessed CGs produced by the National Institute for Health and Care Excellence (NICE) to identify and describe updated recommendations and to investigate potential factors associated with updating. Also, we evaluated the reporting and presentation of recommendation changes. METHODS: We performed a descriptive analysis of original and updated CGs and recommendations, and an assessment of presentation formats and methods for recording information. We conducted a case-control study, defining cases as original recommendations that were updated ('new-replaced' recommendations), and controls as original recommendations that were considered to remain valid ('not changed' recommendations). We performed a comparison of main characteristics between cases and controls, and we planned a multiple regression analysis to identify potential predictive factors for updating. RESULTS: We included nine updated CGs (1,306 recommendations) and their corresponding original versions (1,106 recommendations). Updated CGs included 812 (62%) recommendations 'not reviewed', 368 (28.1%) 'new' recommendations, 104 (7.9%) 'amended' recommendations, and 25 (1.9%) recommendations reviewed but unchanged. The presentation formats used to indicate the changes in recommendations varied widely across CGs. Changes in 'amended', 'deleted', and 'new-replaced' recommendations (n = 296) were reported infrequently, mostly in appendices. These changes were recorded in 167 (56.4%) recommendations; and were explained in 81 (27.4%) recommendations. We retrieved a total of 7.1% (n = 78) case recommendations ('new-replaced') and 2.4% (n = 27) control recommendations ('not changed') in original CGs. The updates were mainly from 'Fertility CG', about 'gynaecology, pregnancy and birth' topic, and 'treatment' or 'prevention' purposes. We did not perform the multiple regression analysis as originally planned due to the small sample of recommendations retrieved. CONCLUSION: Our study is the first to describe and assess updated CGs and recommendations from a national guideline program. Our results highlight the pressing need to standardise the reporting and presentation of updated recommendations and the research gap about the optimal way to present updates to guideline users. Furthermore, there is a need to investigate updating predictive factors.