The effect of prolonged local pressure on the skin microcirculation was investigated in the regions of the sacrum and gluteus maximus muscle to determine the aetiology of pressure sores. Thirty normal subjects (15 female, 15 male) were investigated. The subjects were divided into three age groups: group 1, less than or equal to 35 years; group 2, 36-64 years; and group 3, greater than or equal to 65 years. Local pressure was applied with a specially designed instrument according to the sequence 0----110----0----110----0----mmHg. Skin blood cell flux (SBF) was measured with a laser-Doppler technique and the local skin temperature measured with a thermistor. No significant differences were seen in SBF due to sex or age. However, differences between the two areas studied were observed. SBF was maximal over the sacrum at 12-50 mmHg (1.6-6.7 kPa) applied pressure. With further pressure increases, the SBF signal decreased successively, reaching minimum level at 110 mmHg (14.6 kPa), where it was approximately 43% below the initial value. The SBF in the gluteus region showed a more stable pattern, with a maximum SBF at 13-60 mmHg (1.7-8.0 kPa). At both locations, an increased SBF at zero pressure was seen when the pressure was decreased from 110 to 0 mmHg. The skin temperature (n = 7) increased by 2.7 degrees C (range 1.9-3.5 degrees C) over the gluteus and by 1.3 degrees C (range 0.8-2.5 degrees C) over the sacrum. This increase was more rapid over the gluteus region. Concomitantly a temperature-dependent increase of SBF could be seen. It is concluded that a cause for the greater frequency of pressure sores over the sacrum than over the gluteus region is the comparatively poorer regulation of microvascular flow in this area.