<?A3B2 twb=.4w?>The Observatorio del Roque de los Muchachos (ORM), in the Canary Islands (Spain), was one of the candidates to host the future European Extremely Large Telescope (E-ELT) and is the site of the Gran Telescopio Canarias (GTC), the largest optical infrared facility to date. Sky transparency is a key parameter as it defines the quality of the photometry to be acquired in the astronomical observations. We present a study of the atmosphere extinction at the ORM, carried out after analysis of a database spanning more than 20 yr, to our knowledge, the longest and most complete and homogeneous in situ database available for any observatory. It is based on photometric measurements in the V band and r band (transformed to the V-band extinction coefficient k) using the Carlsberg Meridian Telescope (CMT). Clear seasonal variations that repeat yearly are observed. The median value of k is 0.13 mag airmass; the mean value has a maximum in the summer months (June-September), corresponding to the season with maximum frequency of nights affected by dust or cirrus (˜29% in summer, but only ˜13% during the rest of the year). Two volcanic eruptions took place during the database baseline, which has enabled the study of the impact of volcanoes on the global atmosphere extinction. For the 5 yr of available information, we have estimated the average monthly weather downtime from the CMT data log, obtaining a result (20.7%) in reasonable agreement with earlier studies. The main conclusion of our study is that there is no significant evidence from the CMT data for any secular changes in k over the 20 yr database baseline.