While different Pediatric ALL study groups have used varying definitions of early response (BM vs. PB, prophase vs. day 7 vs. day 14), all agree that it provides critical prognostic information. Bone Marrow aspiration and biopsy (BMA/B) is an invasive procedure requiring sedation or anesthesia, and an early/mid induction specimen may be difficult to interpret even by experienced hematopathologists. In this study we attempted to determine if there was a concordance between peripheral blood blast (PBB) clearance and the findings of the day 14 BMA/B, and whether day 7 PBB count could reliably replace a mid induction BMA/B. Clinical data for newly diagnosed pediatric (<14 years) ALL patients between January 1999 and December 2001 were retrieved from our prospective database. Day 14 BMA/B slides were reviewed independently by two hematopathologist. For the total 165 patients, median age was 4 years, 53.9% were boys. Complete information was available for 151 of these patients and further analysis is based on this number. 124 (82.1%) were treated with 4 agents while the remainder received a 3-drug induction. 23 (18.5%) had positive PBB on D7, and 21 (13.9%) had >5% blasts in the D14 BMA/B. The D7 PBB count could positively and negatively predict the D14 BMA/B 71.9% and 89.4% of the times, respectively. In conclusion, when the D14 BMA/B is used as a measure of early response, an absence of D7 PBB can reliably predict a negative BM, however persistence of PBB does not necessarily predict a sub-optimal BM response to early therapy. Therefore, patients without PBB on D7 may not require BMA/B on D14, therefore avoiding an invasive procedure for this group of patients.