The discontinuities in the Earth's upper mantle at 410 km and 660 km are frequently considered as phase changes, which are exothermic and endothermic, respectively. From this follows that at laterally changing temperatures both discontinuities move in opposing vertical directions. That means they are negatively correlated. We attempt to test this hypothesis using travel times of seismic waves converted at both discontinuities underneath seismic stations. The broadband stations used are located in areas of high (e.g. western US) and low heat flow (e.g. Central Asia) [Sclater et al., 1980>. We observe a clear positive correlation of the conversion times of both discontinuities, which can however be explained with known lateral velocity changes above 410 km. From the scatter of our data we conclude that the limit of a possibly existing and negatively correlated topography of the two discontinuities is about 28 km. This is in contrast to recently claimed observations of about 35 km anticorrelated topography in the western Jordan, 1991; Stammler & Kind, 1992>. ¿American Geophysical Union 1992