An 11 year-old girl was diagnosed as acute lymphoblastic leukemia (ALL) on November 26th 1979 and was induced into complete remission with vincristine and prednisolon. After consolidation therapy with daunomycin, vincristine and prednisolone, she developed pneumonia on January 21st 1980. No cause of pneumonia was found by sputum culture or serologic tests. Treatment with cefmetazol (CMZ), sulbenicillin (SBPC) and minocycline (MINO) was not effective but 9 g/day of LCM made a remarkable effect. Indirect immunofluorescence assay of antibody showed x 512 titers on January 22nd and in her recovery period, the titers showed eight times increased up to x 4,000. The diagnosis of Legionellosis was made on CDC's criteria. She recovered completely with sequential lincomycin (LCM) and erythromycin (EM) therapy. On October 1989, she is still in the first complete remission of ALL for more than 8 years.