Results of ambient vibration tests of the eight-story reinforced-concrete hospitalization tower of the Mexicali General Hospital are described. The structure suffered some damage during the November 1987 Superstition Hills earthquakes. The tests were conducted in April and August 1989 before and after major alterations of the building were made. The frequencies, modal damping ratios and mode shapes of some of the longitudinal, transverse and torsional modes were determined for the April and August 1989 conditions. It was found that the removal of the facade of the building resulted in a reduction of the longitudinal stiffness of the structure of the order of fifty percent. Measurements of the translation and rocking of the base indicate that soil-structure interaction effects play a moderate role in the transverse response of this structure and a negligible role in its longitudinal response.