The purpose of this paper is to describe the statistical impact of the Tenth Revision of the International Classification of Diseases (ICD-10) on cause-of-death data for the United States. ICD-10 was implemented in the U.S. effective with deaths occurring in 1999. The paper is based on cause-of-death information from a large sample of 1996 death certificates filed in the 50 States and the District of Columbia. Cause-of-death information in the sample includes underlying cause of death classified by both ICD-9 and ICD-10. Preliminary comparability ratios by cause of death presented in this paper indicate the extent of discontinuities in cause-of-death trends from 1998 to 1999 resulting from implementing ICD-10. For some leading causes (for example, septicaemia, influenza and pneumonia, Alzheimer's disease, and nephritis, nephrotic syndrome and nephrosis) the discontinuity in trend is substantial. Results of this study, although preliminary, are essential to analysing trends in mortality statistics between ICD-9 and ICD-10. In particular, the results provide a means for interpreting changes between 1998, which is the last year in which ICD-9 was used, and 1999, the year in which ICD-10 was implemented for mortality in the United States. Published in 2003 by John Wiley & Sons, Ltd.