This dataset provides the details of all sites which have been monitored as part of the UK Butterfly Monitoring Scheme (UKBMS). Data includes the location within the UK, the length and width of the line transect on each site, and how long the transect has been monitored. The UKBMS started in 1976 with fewer than 50 sites. The number of sites monitored each year has increased to over a thousand since 2008. There is turnover in sites monitored each year and details of the first and last year in which each site was surveyed are given. The majority of site data is provided by recorders at the time a transect is created. The majority of these recorders are volunteers. The Centre for Ecology & Hydrology (CEH) and Butterfly Conservation (BC) collate the data and the UKBMS is funded by a consortium of organisations led by the Joint Nature Conservation Committee (JNCC). Full details about this dataset can be found at https://doi.org/10.5285/9b7fa1d0-2580-46a5-a2d6-b17d30fd7723