The data above were extracted from the Essential Science Indicators database of Thomson Reuters. This database, currently covering the period January 1998 through December 2008, surveys only journal articles (original research reports and review articles) indexed by Thomson Reuters. Articles are assigned to a category based on the journals in which they were published and Thomson Reuters’ journal-to-category field-definition scheme. Both articles tabulated and citation counts to those articles are for the period indicated. Naturally, institutions publishing larger numbers of papers have a greater likelihood of collecting more citations than those publishing fewer papers. This ranking is by citations per paper (impact) among those European institutions that have collected 100,000 or more citations in clinical medicine during the period. For papers with multiple institutional addresses, each institution receives full, not fractional, citation credit. Essential Science Indicators lists institutions ranked in the top 1 per cent for a field over a given period, based on total citations. For the current version, 2,924 institutions are listed in the field of clinical medicine, meaning that a total of 292,400 institutions were surveyed to obtain these results. Of the 2,924, a total of 98 institutions collected 100,000 or more citations. The ranking by citations per paper (impact) seeks to reveal heavy-hitters based on per-paper influence, not mere output. Also appearing is the world rank by impact for each of these European institutions. For more information on Thomson Reuters’ Essential Science Indicators, see http://scientific.thomsonreuters.com/products/esi.