Registered Nurses (RNs) are the largest group of health care professionals in the United States. In 1998 there were more than 2 million employed RNs in health care organizations. Between 1983 and 1998 the average age of working RNs increased by 4.5 years. Current explanations of this phenomenon highlight demographic, social and educational forces with limited empirically based analysis of the causes and implications of this aging workforce. This article identifies and assesses important sources of changes in the age distribution and total supply of RNs and projects the future age distribution and total RN workforce up to the year 2020. The authors used a retrospective cohort analysis of employment trends of recent RN cohorts over their lifetimes based on U.S. Bureau of the Census Current Population Surveys for the years 1973 to 1998. They then used recent workforce trends to forecast long-term age and employment of RNs. Participants of the study included 60,386 employed registered nurses, aged 23 to 64 years. Based on the authors' research, the primary factor that has led to the aging of the RN workforce appears to be the decline in younger women entering the nursing profession during the past 20 years. According to the authors, if this trend is not reversed, the RN workforce will continue to age, decrease in overall size, and fail to meet projected long-term workforce requirements.