Abstract Synchrotron X-ray diffraction measurements above 100 GPa indicate that Pb at 300 K undergoes a high-pressure phase transition from h.c.p. to b.c.c. structure. The density difference between the two phases is very small (∼0.3%), and thus the metastable two-phase region is large, from 74 to 180 GPa. The isothermal equation of state of Pb is measured to 238 GPa. The observation of the phase transition is in good agreement with theoretical predictions. Recent shock-wave studies and theoretical studies predict lower pressures at high compressions than are indicated by the present measurements.