ATL-104 is a potent mitogen for epithelial cells of the gastrointestinal tract. In animal models, ATL-104 aids regeneration of the gastrointestinal tract after treatment with chemotherapeutic agents. The effect of ATL-104 on mucositis in patients requiring high-dose melphalan or BEAM before peripheral blood SCT (PBSCT) was investigated in a randomized, placebo-controlled, double-blind, two-part study. Patients were randomized to ATL-104 (50, 100 or 150 mg) or placebo once daily for 3 days before chemotherapy and 3 days after PBSCT. Part one of the study was a dose-escalation design; part two was a parallel group design using all three ATL-104 doses. Patients were followed up for 28 days post-treatment. Severity of signs and symptoms were assessed and used to calculate scores for standard toxicity rating scales (WHO, Western Consortium for Cancer Nursing Research (WCCNR)). Sixty-three patients were treated. Treatment with ATL-104 substantially reduced the median duration of severe oral mucositis (WHO grade 3 or 4) compared with placebo (median duration: ATL-104 groups 2 or 3 days, placebo 10.5 days). The effect of ATL-104 on the incidence of severe oral mucositis was inconclusive. Similar results were obtained using the WCCNR Scale. Adverse events (AEs) were predominantly mild or moderate in intensity. Gastrointestinal AE were most common.