There has been little empirical documentation of the acute effects of bone marrow or stem cell transplant (BMT) on children. In the present study, the responses of 153 children undergoing BMT were assessed in a prospective, longitudinal design. Children were assessed at the time of admission for transplant, then underwent weekly assessments to week +6, followed by monthly assessment to month +6. Data were obtained both by parent report and patient report (for patients age 5 and up) using the BASES scales. The major findings are: (1) children undergoing BMT enter the hospital with an already heightened level of distress (defined by high levels of somatic symptoms and mood disturbance, and low levels of activity) that increases dramatically following conditioning, reaching a peak approximately 1 week following transplant; (2) this increased distress is transient, declining rapidly back to admission levels by week +4 to week +5, followed by a further decline to presumed basal levels by months 4-6; and (3) the trajectories of distress depicted by both parent and child report are remarkably similar, each providing confirmatory support for the validity of the findings. These findings confirm a number of widely held clinical impressions that had not previously been documented empirically, and point to the need for new interventions or more intensive approaches to supportive care aimed at reducing levels of distress during the acute phase of transplant.