F region peak heights, derived from ionospheric scaled parameters through 38-year data series from both Argentine Islands (65 ¿S, 64 ¿W) and Port Stanley (52 ¿S, 58 ¿W) have been analyzed for signatures of secular change. Long-term changes in altitude, which vary with month and time of day, were found at both sites. The results can be interpreted either as a constant decrease in altitude combined with a decreasing thermospheric wind effect or as a constant decrease in altitude which is altitude-dependent. Both interpretations leave inconsistencies when the results from the two sites are compared. The estimated long-term decrease in altitude is of a similar order of magnitude to that which has been predicted to result in the thermosphere from anthropogenic change related to greenhouse gases. Other possibilities should not, however, be ruled out. ¿ 1998 American Geophysical Union