Serial contrast magnetic resonance imaging (MRI) has played an increasingly important role in understanding natural-history and early-treatment trials of multiple sclerosis patients. The purpose of this study is to determine whether the serial administration of gadopentetate dimeglumine at the conventional dose has any demonstrable effect on routine hematologic or serum chemistries. This study followed 56 patients with multiple sclerosis in a longitudinal natural-history trial using contrast-enhanced MRI scans over a four-year period between 1988 and 1993. Patients received between 3 and 53 doses of gadopentetate dimeglumine at 0.1 mmol/kg intravenously. A retrospective review of regular blood screening tests over this period identified no significant effect either on routine hematologic studies, as defined by complete blood count (hemoglobin, hematocrit, platelet and white blood cell counts, and mean corpuscular volume); standard serum chemistry studies, including electrolytes (sodium, potassium, chloride) and renal and liver function tests; or serum iron profiles. We conclude, therefore, that serial contrast-enhanced MRIs can be used safely as an outcome measure for Phase I/II evaluations of new therapies for multiple sclerosis.