National payment policies target a hemoglobin range of 11 to 12.5 g/dl for patients with ESRD. However, clinical complications and provider practices may contribute to wide fluctuations over time. This study evaluated the frequency with which patients maintain stable hemoglobin levels below, within, and above the Centers for Medicare & Medicaid Services target range and assessed patterns of hemoglobin level change that resulted in large fluctuations across the target range during a 6-mo period. All hemodialysis patients who survived the first 6 mo of 2003, had Medicare as primary payer, and had Medicare outpatient erythropoietin claims in each of the first 6 mo of 2003 (n = 152,846) were studied. Six patient groups were defined on the basis of patterns of hemoglobin level fluctuation: Consistently low (<11 g/dl), consistently target range (11 to 12.5 g/dl), consistently high (> or =12.5 g/dl), low-amplitude fluctuation with low hemoglobin levels, low-amplitude fluctuation with high hemoglobin levels, and high-amplitude fluctuation. Only 10.3% of patients maintained stable hemoglobin levels during the 6 mo and only 6.5% in the target range. The consistently low group had the highest percentage of hospitalizations and the highest number of comorbid conditions. High-amplitude fluctuation was the most common pattern (39.5%), with hemoglobin levels falling below and rising above the target range during the 6-mo period. Hemoglobin levels in almost 90% of patients are in some degree of flux at any point in time, and the fluctuation is highly associated with clinical complications and provider practices.