The use of irrigating fluid containing a small amount of ethanol allows fluid absorption to be monitored by means of expired-breath tests during transurethral resection of the prostate (TURP). There is, however, no consensus concerning how the breath ethanol data should be converted into the volume of irrigant absorbed. The present study examines the use of the product of concentration and time (area under the curve) to predict how much fluid is absorbed. Intermittent infusions of ethanol in 5 volunteers showed that the relationship between area under the curve and ethanol load was best described by a polynomial expression. This was also applicable to the uptake of small volumes of irrigating fluid during 16 TURPs. When patients with large-sized absorption ( > 1,000 ml) were also considered, however, the precision was better when the absorbed volume was predicted by a multiple linear regression formula where the ethanol level in the exhaled breath at the end of any 10-min period during TURP and the time during which ethanol had been detected served as independent factors.