BACKGROUND AND OBJECTIVE: A preference for English-language sources during determination of Journal Impact Factors (IF) was discussed, IF being published in the annual Journal Citation Reports (JCR). The JCR are derived from data in Science Citation Index (SCI). The aim of this study was, therefore, (i) to review publication countries and languages in JCR, (ii) publication languages in SCI in comparison to further recognised medical bibliographic databanks. METHODS: Searching (i) countries and languages in JCR Science-Editions 1997 and 1998, (ii) language distributions in publication years 1995 - 2000 in bibliographic databanks SCI, MEDLINE (ME) and EMBASE (EM). RESULTS: (i) Almost 70 % journals in JCR 1997 and 1998 were published in USA, United Kingdom, or The Netherlands. Of two language options present, a number of English-classified journals contained >90 % articles in other languages, whereas >90 % publications in English could occur in Multi-Language (ML) journals, thereby complicating statistical comparisons. 83,9 % JCR-periodicals in 1997 and 85,6 % in 1998 were classified English. English/ML ratios increased exponentially with increasing IF. (ii) 95,5 % of the articles documented 1995 - 2000 in whole SCI and in our constructed SCI segment >>Medicine and related areas<< were written in English, compared to 88,5 % in ME and 89,8 % in EM. The SCI Medicine segment was 15 % more comprehensive than either MEDLINE or EMBASE. Highly significant differences of language distributions in SCI vs. MEDLINE and especially SCI vs. EMBASE were observed. Retrieval rates in SCI of German-, French-, Japanese- and Chinese-language medical papers published in 2000 were impressively augmented by EMBASE and MEDLINE. CONCLUSIONS: (i) Anglo-American publishers' countries and English-language journals prevail in JCR with respect to numbers and IF levels. Publication language English favours citation frequency. (ii) Of databanks studied, SCI shows a maximum preference for English-language sources, thereby causing an English Language Bias during IF derivation.