Deterioration of final effluent from small-scale onsite wastewater treatment plants (SOWTPs) for individual houses frequently occurs due to their malfunction if the family has a patient with diabetes. However, since little is known about the causes of this deterioration, no effective countermeasures have been taken. To understand the causes, we analyzed and evaluated the effects of glucosuria on the performance of the plants. The BOD of glucosuria was found to be markedly high, and the BOD in the urine of the patients with severe diabetes (3.0% or more saccharides) was about 10 times higher than that of the normal urine. The BOD load in the urine of the severe diabetics was calculated to be 61-142g/cap/day, equivalent to 20-47 times higher urinary BOD load of normal persons. The BOD load of night soil containing the urine of severe diabetics was estimated to be 71-152 g/cap/day, which was 5.5-12 times higher than the corresponding BOD of normal person of the night soil. It was suggested that the malfunction of the plants by glucosuria was caused by a significant increase in the BOD load.