The tropical tropopause region (14-18km altitude) plays an important role in the climate system, but the technical difficulties of making measurements in this region are severe. This paper reports observations of very thin tropical tropopause cirrus clouds made using a new lidar instrument carried on long-duration balloon flights, lasting several weeks and travelling about 20000km, from the Indian Ocean to the Central Pacific. The sensitivity of the new instrument reveals that clouds are much more frequent in this part of the atmosphere than had been identified previously. The quantitative significance for the large-scale climate system, e.g. for the radiation balance, is yet to be assessed, but it is clear that these observations will be a valuable resource for scientists studying this truly remote part of the atmosphere.