A distinctive cerebellar ocular motor disorder, macrosaccadic oscillation, evolved simultaneously with an acute cerebellar syndrome in 4 patients, 2 with haemorrhagic metastatic melanoma deep in the vermis, a third with a presumed cerebellar haematoma and a fourth with focal demyelinating disease. Ocular oscillations were conjugate, horizontal, symmetrical, occurred in bursts of several seconds duration, had amplitudes of 30 degrees to 50 degrees, and were evoked whenever the patient attempted to shift visual fixation or pursue a moving target. Photo-electric recordings in one patient with tumour defined features of this disorder of saccadic eye movement: (i) oscillation was composed of saccades, (ii) frequency was 2 Hz, (iii) bursts occurred with amplitude first increasing and then decreasing, (iv) intervals between beginnings of saccades averaged 260 ms and (v) eye position did not exhibit systematic drift during the intersaccadic period. These features documented the inreased gain and instability of the visually guided saccadic system. By using increased feed-forward gain in a sampled-data control model we simulated the pattern of macrosaccadic oscillation. We belive that the acute loss of the calibrator function of the cerebellum accounts for the gain abnormality underlying macrosaccadic oscillation.