Congenital nystagmus (CN) may be due to an instability of the neural integrator responsible for gaze holding. This longitudinal study tests that hypothesis, investigates the saccadic instabilities of relatives, and assesses the effects of afferent stimulation on both the CN and the coexisting gaze-holding failure. We recorded four siblings who had CN and gaze-holding failure while fixating in primary position and lateral gaze. In lateral gaze, the CN waveforms were superimposed on the centripetal drift caused by the gaze-holding failure; the drift time constants ranged from 300 to 1,450 msec. CN waveforms lacked extended foveation periods. Saccadic instabilities were present in the father and two clinically unaffected siblings; the mother's eye movements were normal. We conclude that CN in the subjects of this study, and in others with idiopathic CN, is not due to gaze-holding abnormalities, and we speculate that development of the fixation reflexes that produce CN foveation periods requires some minimal foveation interval during which the target image is in the foveal area with low retinal slip velocity and acceleration.