12 AIDS/ARC patients with or suspected of Pneumocystis carinii pneumonia were treated with co-trimoxazole and received supplementary folic or folinic acid to avoid peripheral blood cytopenia. Most patients developed decreased numbers of neutrophils and hemoglobin while receiving co-trimoxazole. Supplementary folate/folinate could not abolish the drug-induced cytopenia. Routine prescription of folinic acid is not recommended. Folic acid is cheap and may be beneficial and should be prescribed.