Staggerer (sg) is a neurological mutant mouse in which the cerebellar granule cells degenerate after migration to the internal granule cell layer. In addition, the Purkinje cells are abnormal, being ectopic, smaller in size and without tertiary branchlet spines. In this study we report two new observations on this mutant: (1) the effects of the mutation show a regional variation in severity along the mediolateral axis. This variation is seen in the cross-sectional size of the tissue, the extent of cortical folding, as well as the density and cytological appearance of the medium-to-large cortical neurons (MLNs); (2) cell counts were done of 30-day-old mutants and of wild-type controls. The counts revealed that three-quarters of the MLNs of the cerebellar cortex are missing in staggerer. These findings cannot exclude the possibility that Golgi as well as Purkinje cells are absent since the two cannot be distinguished in staggerer. Depending on the number of Golgi cells present, between 60% and 90% of the wild-type number of Purkinje cells is missing in staggerer. The results are discussed in terms of their implications for other studies and for locating the site of staggerer gene action.