The pressure in, and thus the size of, a bubble in a lava flow is determined by the atmospheric pressure and the hydrostatic pressure of the overlying lava. If atmospheric sea-level pressure is known (or assumed), vesicle size distributions in basalt flows can thus be used as an indicator of the palaeoelevation of emplacement(1). Here we show by analysis of the vesicle size distribution of basalt samples collected from the summit and base of Mauna Loa volcano in Hawaii that the technique provides estimates of ambient pressure that are accurate to within 0.1 bar, and thus estimates of elevation with a resolution of about 1,400 m. This palaeoaltimetric technique should find useful application in palaeogeographical reconstruction(2) for times when sea-level pressure was similar to the present value. (Conversely, if the elevation of emplacement is known, our technique should be able to provide a measure of sea-level palaeopressure.) Unlike palaeobotanical methods for estimating elevations(3-6), this method does not have to assume a relationship between temperature and altitude; on the other hand, its use is restricted to lava flows that have had a simple emplacement history.