The exact temperature of the head of an unheated Fleisch pneumotachograph (PT) during recording is not known, and variation in its temperature may lead to errors in measuring spirometric indexes. We measured PT head temperature during blows from five normal subjects, recorded by using a PT with and without an upstream heat exchanger to condition the air to the ambient temperature that was set in a climate chamber. Group mean (+/- SD) temperature of a thermocouple (TC) placed inside the PT head was 11.8 +/- 1.9 degrees C with 7 degrees C ambient, 25.4 +/- 1.3 degrees C at 23 degrees C, and was 37.2 +/- 0.3 degrees C at 37 degrees C. The between-subject range of temperature for this TC was 7.5 degrees at 7 degrees C, 5.5 degrees at 23 degrees C, and 1.1 degrees at 37 degrees C. The mean within-subject within-blow variation of temperature for this TC was 10.0 degrees and 3.3 degrees C for ambient of 7 degrees and 23 degrees C, respectively. At the usual ambient temperature in a laboratory, these differences in temperature lead to a 3.6% between-subject bias in recording, and the within-subject differences lead to 2.6% underreading of peak expiratory flow and a 0.5% overreading later in the blow, which makes ATPS-to-BTPS correction erroneous or difficult to perform. With the use of an upstream heat exchanger, the group mean temperature was 8.7 +/- 0.4 degrees, 23.2 +/- 0.2 degrees, and 37.1 +/- 0.2 degrees C at the three ambient temperatures, respectively, and the within-subject within-blow variation was reduced to < 1 degrees C. A heat exchanger placed upstream of the PT satisfactorily conditioned expired air to the ambient temperature and removed the error.