PURPOSE: Critically ill neonates are frequently transfused with packed red cells. Some of these transfused neonates also need chromosome analysis. There is a long-standing tradition in pediatrics of not performing chromosome analysis after transfusion. We wished to determine whether transfusion with packed red cells affect the cytogenetic results in neonates. METHOD: The medical records of all neonates at the Medical College of Georgia who had had chromosome analysis between June 1995 and June 1998 were reviewed. Ten neonates had received transfusion prior to cytogenetic testing. Of these 10 infants, two had been transfused two or more times. Routine cytogenetic analysis of 20 metaphases at 550-band level had been performed on all 10 patients. Heteromorphic markers were compared in 10 randomly selected metaphases for any discrepancy. To determine whether there were theoretical reasons to delay chromosome analysis in transfused neonates, samples of irradiated, and/or filtered, and nonfiltered blood were obtained from the blood bank and analyzed for the presence of lymphocytes. RESULTS: Prior transfusion did not affect karyotype results. A nonmosaic abnormal karyotype was found in 3 of the 10 patients. A fourth patient's karyotype was 45,X/47,XXX. This mosaicism was constitutive and consistent as demonstrated by a follow-up chromosome analysis. All other abnormal karyotypes were consistent with the dysmorphic phenotype. Randomly selected metaphases did not show any differences in the identifiable heteromorphic markers in all 10 patients. Although there was a 50% chance of patients receiving blood from a donor of opposite sex, there were no instances in which cells with a karyotype of the opposite sex were found in the patients' blood. The irradiated and filtered cultured donor blood samples did not show any metaphases. However, metaphases were seen in the cultures from nonfiltered and nonirradiated donor blood. CONCLUSIONS: Based on these results one does not need to delay karyotyping babies who have had blood transfusions. Packed red cell transfusion in newborns does not compromise the accuracy of chromosome analysis in our study even with multiple transfusions.