A 77-year-old man with chronic obstructive pulmonary disease was treated with low-dose methotrexate (7.5-15 mg per week). After 15 months a diagnosis of urothelial carcinoma of the bladder was made; after a further 6 months pneumonitis and pancytopenia developed. The patient died due to massive pulmonary hemorrhage. A malignant teratoma was diagnosed in a 65-year-old asthmatic man 16 months after initiation of methotrexate therapy (15 mg per week). The patient died 4 months later due to fulminant progression of the neoplasm. A third malignant neoplasm (dermal squamous cell carcinoma) was seen in a 64-year-old woman with rheumatoid arthritis after 13 months treatment with 7.5 mg methotrexate per week. These three cases, while obviously not proving a causal relationship between long-term treatment with low-dose methotrexate and development of malignant neoplasm, do call for stringent treatment criteria, close surveillance, and prospective studies.