A 72-year-old woman with diabetic nephropathy was hospitalized with peripheral edema in the extremities and weight increase. After diuretics and human serum albumin administration, her condition improved. From the 15th day she had run a subfever and her breathing was diminished in the left lower lung field. A plain chest X-ray film showed pleural effusion over the left lung field. The fluid was exudative. Fluid cultures were negative. A tuberculin reaction was negative. Polymerase chain reaction method disclosed mycobacterium avium complex, indicating rare pleuritis due to mycobacterium avium complex. Eighteen days after chemotherapy, pleural effusion disappeared. Although her hemoglobin A1c (HbA1c) levels were maintained from 6.0 to 6.5% over 4 years, urinary albumin excretion levels and serum creatinine levels increased, indicating deteriorating diabetic nephropathy. Serum albumin levels remained low (3.3-3.6 g/dl). Malnutrition, impaired cellular immunity and apparently abnormal microvascular circulation due to diabetes mellitus may consequently have induced pleuritis due to mycobacterium avium complex.