This study is designed to estimate the prevalence of and gain further insight into the characteristics of the chronic kidney disease (CKD) population in a large US health maintenance organization (HMO) to better understand the CKD population in the United States overall. Analyses were performed using data from a staff and network model HMO in the southwestern United States with more than 150,000 members per year during 1994 to 1997. The estimated prevalence of CKD in the HMO population varied from 0.4% to 7.1%, depending on the definition of CKD used. Regardless of the definition, CKD was more common in men compared with women and in patients with diabetes mellitus and/or hypertension. Applying the age- and sex-specific prevalence rates in the HMO to the US population in 1990, we estimate there were approximately 9.1 million Americans with at least one elevated sex-specific creatinine (Cr) value and approximately 4.2 million Americans with at least two elevated Cr values separated by 90 days or greater, a more rigorous definition of CKD. From these results, it is apparent that there are a large number of patients in the United States with CKD. Most have not been identified because screening for CKD generally is not performed. Considering the high prevalence of CKD and the high cost and clinical morbidity associated with end-stage renal disease (ESRD), it is clear that CKD is an important public health problem. Early identification of patients with CKD would allow treatment that could slow the progression to ESRD, improve clinical outcomes, and constrain the growth of costs in the ESRD program. The time has come for a structured public and professional educational program to address this serious condition.