A study by Gunther Eysenbach in PLoS Biology suggests that open access articles “are more immediately recognized and cited by peers than non-OA articles published in the same journal.” Therefore, he concludes, “OA is likely to benefit science by accelerating dissemination and uptake of research findings.”
The study consisted of comparing citations among OA and non-OA articles published June 8, 2004 – December 20, 2004, in PNAS: Proceedings of the National Academy of Sciences. (Thanks to Don Marti for the link.)