Nimustine (ACNU) is an alkylating agent of the nitrosourea and can be an antineoplastic agent in dogs. But, there has been no report on its dose-limiting toxicity (DLT) in dogs. This study was a phase I dose-escalation clinical trial to determine the maximum tolerated dose (MTD) and DLT of ACNU in tumor-bearing dogs. The starting dosage was 25 mg/m2, and subsequent dosages were administered in increments of 5 mg/m2 in cohort of 3 dogs. Eight dogs were included and the MTD was determined to be 25 mg/m2, DLT was neutropenia and the optimal interval was considered to be 21 days. The data herein provides a basis for the subsequent phase II trial of ACNU in dogs.