Secondary bone marrow fibrosis (BMF) is associated with many disease conditions in children, but its prevalence and characteristics have not been well elucidated. We present our experience with pediatric secondary BMF, in an attempt to characterize it in terms of underlying diagnoses, severity, and outcome. A retrospective chart review of patients diagnosed with secondary BMF by bone marrow aspirate and biopsy between January 1984 and April 2011 showed a total of 214 patients, the majority (67.1%) of whom had an underlying oncologic disease. At diagnosis, 87 patients (39.7%) had mild, 51 (23.3%) had moderate, and 33 (15.1%) had marked BMF; it was not quantified in 48 (21.9%) patients. An underlying oncologic disease was more frequently associated with marked fibrosis compared with hematologic and miscellaneous diagnoses. Follow-up posttreatment bone marrow aspirate assessments were available for 117 patients. The outcome ranges from worsening of fibrosis to complete resolution. A majority of these children (N=70/117, 60%) showed complete resolution of fibrosis. Of note, 27 patients had marked fibrosis at initial diagnosis and 16 (60%) of them showed complete resolution. These findings underscore the importance of appropriate treatment of the underlying disorder in reversing secondary BMF. Ours is the largest series of pediatric secondary BMF reported.