In order to know the adequate duration of the chemotherapy with two drugs (INH + RFP) for pulmonary tuberculosis with non-cavitary minimal radiological findings (minimal case), 278 cases with minimal lesion which had completed 9 months' chemotherapy, were observed for more than six months up to 5 years (mean duration = 54.4 months). Of them, 60 cases were bacteriologically confirmed by smear and/or culture examination. Many cases showed further improvement in radiological findings even after the end of the chemotherapy. Of 180 cases of initially infiltrative type (GAKKEN B type), 10 cases showed the enlargement of shadow radiologically, but were not regarded as relapsed cases, because they remained bacteriologically negative and the shadow improved in 1-2 months without additional chemotherapy. Only 3 cases (1.1%) were regarded as relapsed cases because of the positive bacteriological conversion and aggravation of the shadow. They were initially sputum negative. It can be concluded that for radiological minimal cases, nine months is enough for the duration of chemotherapy when the INH-RFP regimen is used.