The Landsat Thematic Mapper (TM) offers a means of detecting and monitoring thermal features of active volcanoes. Using the TM, we have discovered a prominent thermal anomaly on Lascar volcano, northern Chile. Data from two short-wavelength infrared channels of the TM show that material within a 300-m-diameter pit crater was at a temperature of at least 380 °C on two dates in 1985. The thermal anomaly closely resembles in size and radiant temperature the anomaly over the active lava lake at Erta'ale in Ethiopia. An eruption took place at Lascar on September 16, 1986. TM data acquired on October 27, 1986, revealed significant changes within the crater area. We find that Lascar is in a much more active state than any other volcano in the central Andes, and for this reason it merits further careful monitoring. Our studies show that the TM is capable of confidently identifying thermal anomalies less than 100 m in size, at temperatures of above 150 °C, and thus it offers a valuable means of monitoring the conditions of active or potentially active volcanoes, particularly those in remote regions.