The objective of this study was to determine if loss of antibodies to measles and to varicella occurs in pediatric solid organ transplant recipients. Antibodies were measured in eight hepatic, four cardiac, and six renal transplant patients who had antibodies to measles prior to transplantation, and in six hepatic, five cardiac, and seven renal transplant patients who had antibodies to varicella prior to transplantation. Within 6 months of the transplant, loss of antibodies occurred in four of 18 patients who were seropositive for measles prior to transplantation (22.2%) and in two of 18 patients who were seropositive for varicella prior to transplantation (11.1%). In conclusion, serology for measles and varicella should be considered annually following solid organ transplantation so that appropriate advice can be given to patients who have seroreverted.