Characteristics and longevity of electronic citations in four leading biomedical journals in Spain. 
We aimed to determine the frequency of use and accessibility over time of electronic citations in Revista Española de Cardiología and 3 other Spanish biomedical journals, and to identify the factors that influence the accessibility and retrievability of these links.  A cross-sectional, descriptive study was performed of all the references in articles published in the first issue of each even year from 2002 to 2012 in selected journals.  Data were collected on the journal, publication, references, and links.  The variables analyzed were the percentage of articles with at least 1 electronic citation, the percentage of electronic citations with respect to the total number of citations in the full sample, and the percentages of link accessibility and retrievability.  We included 377 articles, of which 73 (19.4%; 95% confidence interval, 15.2%-23.5%) had at least 1 electronic citation, and a total of 7077 references, of which 164 (2.3%; 95% confidence interval, 2.0%-2.7%) were electronic citations.  Revista Española de Cardiología had 17 (15.2%) and 38 (1.3%), respectively.  Use of electronic citations significantly increased over time (linear tendency, P < .001); 58 (35.4%; 95% confidence interval, 27.7%-43.0%), links were accessible, and information was retrieved in 55 cases (51.9%; 95% confidence interval, 41.9%-61.9%).  Accessibility significantly decreased over time (linear tendency, P < .001); 45 electronic citations (27.5%; 95% confidence interval, 20.3%-34.6%) had complete additional information.  Retrievability was significantly associated with the amount of additional information (linear tendency, P < .001).  Electronic citations are increasingly used in some Spanish biomedical journals.  Access to electronic citations is lost over time, and the probability of its retrieval is associated with the existence of additional information.