It is hypothesized that health professionals in the United States and the United Kingdom are nationally biased in their citation practices. Articles published in the New England Journal of Medicine and Lancet were used to study citation practices of U.S. and U.K. authors. Percentages of cited references to material published in a specific country were calculated for both the New England Journal of Medicine and Lancet. Using a variation of a citation publication ratio based on Frame and Narin's original ratio, an attempt was made to quantify author bias. To calculate these ratios, values from SERLINE* and the British Library Lending Division were employed to find world journal counts. The results suggest that U.S. authors publishing in the New England Journal of Medicine and U.K. authors publishing in Lancet tend to cite material produced in their own countries more than would be warranted by the amount of material produced by these countries. In addition, these authors cited material produced in non-U.S. and non-U.K. countries far less than the amount of material produced by these countries would indicate.