One hundred ninety-seven children with sickle cell anemia were followed for 4 years at the Wayne State Comprehensive Sickle Cell Center to evaluate the stability of the hematological variables (Hb, Hct, RBC count, MCV %HbF and %HBA2) over time. The mean values of the hematological measurements taken during three separate 16-month invervals were used to represent an individual's values. The correlations of the hematological variables between intervals ranged from a low of 0.46 for %HBA2 to a high of 0.91 for %HbF. Correlations that spanned two intervals (an average of 32 months) were of the same magnitude as those that spanned only one interval (an average of 16 months), suggesting that there was no decrease in the degree of stability of these variables as the time between measurements increased. The stability of the correlations between variables within intervals, and the stability of the coefficients of the first two principal components of the six hematological variables over time suggested that the relationships among variables were also stable. In a recent report [Odenheimer et al, 1983], we used the values of the six hematological variables collected at an individual's first visit to the sickle cell center to identify four hematologically distinct subgroups of children. In the same subgroup in at least two of the three follow-up intervals, suggesting that the factors that contributed to this classification were the result of stable, rather than transient phenomena.