Granulocyte colony stimulating factors (GCSFs) stimulate the level of white blood cells, specifically neutrophils. GCSF support for patients receiving chemotherapy was shown to decrease the rate of fever during low neutrophil count (neutropenia), and in some cancer types may decrease mortality. Pegfilgrastim is a pegylated form of the GCSF named filgrastim. Pegfilgrastim is used to stimulate bone marrow to produce more neutrophils to fight infection in patients undergoing chemotherapy. It has a much longer half-life than the parent filgrastim. It is removed from the body within the neutrophils. According to the American Society of Clinical Oncology 2006 guidelines pegfilgrastim should be given 24 hours after the completion of chemotherapy i.e.before neutrophil count starts to drop. Therefore it is cleared before and after neutropenia. Comparative low quality studies suggest that deferring pegfilgrastim delivery until neutrophil counts start dropping may result in improved its efficacy. This was further tested in a few small randomized controlled trials (high quality studies, considered the "gold standard" of studies) in different settings (including first chemotherapy for lymphoma, and solid cancer) with inconsistent results. Pegfilgrastim (given 24 hours after completion of chemotherapy) is a standard part of any salvage chemotherapy for patient with refractory or relapsed aggressive lymphoma. The investigators plan a randomized controlled trial comparing the efficacy of pegfilgrastim given 72 hours (day +3) vs. 24 hours (day +1) after completion of salvage chemotherapy in patients with refractory or relapsed aggressive lymphoma. The investigators will evaluate whether that change of pegfilgrastim schedule affects the risk of fever during neutropenia, neutrophil count, length of hospitalization, mortality, and safety.