A ground-based millimetre wave radar, AVTIS (All-weather Volcano Topography Imaging Sensor), has been developed for topographic monitoring. The instrument is portable and capable of measurements over ranges up to ∼7 km through cloud and at night. In April and May 2005, AVTIS was deployed at Arenal Volcano, Costa Rica, in order to determine topographic changes associated with the advance of a lava flow. This is the first reported application of mm-wave radar technology to the measurement of lava flux rates. Three topographic data sets of the flow were acquired from observation distances of ∼3 km over an eight day period, during which the flow front was detected to have advanced ∼200 m. Topographic differences between the data sets indicated a flow thickness of ∼10 m, and a dense rock equivalent lava flux of ∼0.20 ± 0.08 m3s−1.