Low dose pulse MTX was associated with the development of pancytopenia in six patients with RA. Two patients died. Factors implicated in the occurrence of this complication were renal impairment in five patients, medication errors by two patients, preexisting marrow injury from occult alcoholism in one patient, and an apparent idiosyncratic reaction to the drug in another. Medication errors were associated with the use of five or more medications, and the unusual schedule of administration of low dose MTX may also have been contributory. From a consideration of the clinical pharmacokinetics of MTX, we suggest other factors that may predispose to the occurrence of marrow toxicity: the presence of hypoalbuminemia, interactions between MTX and other protein bound or weakly acidic drugs, and the repetitive dosing schedule of low dose MTX. Based on our experience, patients with impaired renal function (creatinine greater than or equal to 2.0 mg/dL) should not receive MTX. Renal function should be monitored regularly during treatment with MTX, and blood counts should be observed carefully if a new drug is added or substituted. A 5 mg test dose of MTX before initiating weekly therapy may identify patients with severe hypersensitivity to the drug. The potential risks of using MTX in a patient unwilling to accept blood products should be acknowledged and discussed with the patient. Furthermore, we recommend the use of leucovorin if pancytopenia occurs, even if low or undetectable serum levels of MTX are present.