OBJECTIVE To study the number of authors of papers published in Actas Urologicas Españolas, and to assess the consequences on multiple authorship of the restriction to 6 of the number of authors for paper, established in the instructions for authors since January 1995. METHODS The authors and papers appearing on sections "Originals". "Reviews" and "Case reports" from 1992 to 1995, were counted. RESULTS Five hundred and forty eight papers were reviewed. The mean (+/- standard deviation) number of authors (6.1 +/- 2; 95% IC = 5.9-6.3) didn't show a statistically significant change between the 4 studied years (ANOVA, F = 1.42; p = 0.23). Fifty percent of the Originals and Reviews and 36% of Case reports had more than 6 authors and these proportions didn't change significantly between years (chi 2 = 3.48; p = 0.32), even considering that more than one third of papers published in 1995 were accepted that year. CONCLUSIONS A great number of authors should modify their habits of co-authorship on papers submitted to Actas Urologicas Españolas, in order to fulfill the new instructions for authors.