Organ damage in children with sickle cell anemia [SCA] begins with the spleen. Hydroxyurea [HU] decreases clinical complications and mortality in severely affected adults with SCA, and has proven hematologic benefits in children. To critically assess the efficacy of HU in preventing chronic organ damage, the Pediatric Hydroxyurea Phase III Clinical Trial [BABY HUG], an NHLBI sponsored double-blinded placebo-controlled multi-center trial, was initiated. One objective of the Feasibility and Safety Pilot is to evaluate novel strategies for assessment of splenic function in young children with SCA. To date 23 subjects (13 male; median age 12.9 mos, range 10.3–17.6 mos) have been recruited without regard to disease severity. Pretreatment spleen function determined by Tc-99m sulfur colloid liver-spleen [LS] scan was compared to pocked erythrocyte [PIT] counts and flow cytometric quantitation of Howell-Jolly Bodies [HJB]. Results were correlated with total [Hgb] and % fetal [HbF] hemoglobin, white blood cell [WBC] and platelet [PLT] counts. Splenic uptake of Tc-99m was qualitatively graded as normal, decreased, or absent by two nuclear medicine physicians. Of 17 LS scans reviewed 3 had normal (mean age 12.2 mos) and 14 decreased (mean age 14.6 mos) spleen function. LS scans were also imaged quantitatively by determining the geometric mean total counts over the spleen. Although there was a trend for qualitative LS scan results to discriminate splenic function among patients (p=.08), quantitative spleen counts demonstrated a stronger relationship between lower uptake and reduced splenic function. A logarithmic transformation was applied to each measure (except age) to improve linearity with other variables and stabilize the variance of the transformed data. PIT counts (p<.0001) and WBC counts (p=.023) were significantly linearly associated with age. Age was inversely related to Hgb (p=.005) and %HbF (p=.009), but not associated with PLT (p=.54) or HJB (p=.38). Quantitative spleen counts were related inversely to age (p<.01), PIT counts (p=.02), and WBC (p=.026); linearly to %HbF (p=.0003) and Hgb (p=.04); and had no relationship with HJB (p=.39) or PLT (p=.68). In multivariate analysis with age and PIT counts, the decline in spleen counts had the strongest association with %HbF (p=.006). A PIT count of 3.5%, which classically divides normal from decreased spleen function, separated spleen counts into significantly different groups (p<.001). No similar relationship existed for HbF 25% (p=.059), Hgb 8 g/dl (p=.15), or HJB 300/million rbc (p=.28). These preliminary data indicate that the decline of splenic function with age in young children with SCA can be effectively assessed by multiple techniques in a multi-center study. Compared to the traditional qualitative assessment, quantitative evaluation of the LS scan will allow more informative gradation of the decline in splenic function for the BABY HUG study. Surrogate measures such as PIT counts and %HbF are associated with LS scan results, and may prove to be informative non-invasive markers predictive of splenic function.