An implantable cardiac stimulation system provides capture threshold stability assessment. The system includes an implantable pulse generator that provides pacing stimulation pulses, an implantable lead system that couples the pulse generator to a patient's heart, and a capture threshold test circuit that performs capture threshold tests with the pulse generator and provides capture thresholds at spaced apart times. A capture threshold processor responsive to the capture thresholds provides capture threshold stability indicia. The capture threshold stability indicia may be a threshold stability index equal to the ratio of the peak-to-peak threshold fluctuation and the mean capture threshold. The threshold stability index may be compared to a given standard to determine if the capture thresholds are unstable.