Surface temperatures on a hemisphere-cylinder body with a nose cavity in a Mach 4.9 airflow have been measured using an infrared camera, Fluctuating surface pressures have also been measured at the cavity base. The cavity diameter D was fixed at one-half the cylinder diameter and the length L of the cavity was varied. If the cavity lip is sharp and the cavity is ''shallow'' (0.15 less than or equal to L/D less than or equal to 0.35) or ''very deep'' (L/D greater than or equal to 1) an axisymmetric, nominally steady ''cool ring'' forms on the external surface downstream of the lip. Flow visualization shows that the cool ring is caused by separation at the lip. Rounding the cavity lip eliminates or reduces separation and temperatures return to levels characteristic of the model without the cavity. For ''intermediate deep'' cavities (0.49 less than or equal to L/D less than or equal to 0.70) the cavity pressure signals switch from a low-amplitude to high-amplitude level at random intervals resulting in an unstable, nonaxisymmetric temperature field downstream of the lip. Changes in cavity base shape from spherical to flat have little effect on the temperature history for shallow and very deep cavities, whereas for intermediate depth cavities the effects are more significant.