Ionospheric perturbations by natural geophysical activity, such as volcanic eruptions and earthquakes, have been studied since the great Alaskan earthquake in 1964. Measurements made from the ground show a variation of the critical frequency of the ionosphere layers before and after the shock. In this paper, we present an experimental investigation of the electron density variations before earthquakes when the low-altitude satellite TOPEX-POSEIDON is above active seismic regions. Fluctuations in the integrated electron density or TEC (Total Electron Content) between the satellite and the ground are measured to correct the altimetry measurement. The satellite was launched on August 10, 1992 and an analysis of the TEC fluctuations above the main earthquakes (Ms>5) is performed up to the end of 1995. Due to different constraints, the number of earthquakes considered in this study is only 706. Increases as well as decreases of TEC are observed above some epicentres. Three events are detailed and a statistical study is performed with the DORIS data. The corresponding number of TEC perturbations correlated in space and in time (up to 48 h before these earthquakes) is 238, i.e., 34% of the cases. A comparison is made with TEC data randomly selected during the lifetime of the satellite, and ionospheric perturbations are observed in 16% of the cases. This study indicates that it is possible to use TEC measurements to survey these phenomena.