ABSTRACT.Recent studies of factors affecting the P100 latency of the pattern visual evoked potential (PVEP) have questioned the earlier assumption that gender has a statistically significant effect. Guthkelch et al. (1987) reported that head size is an important variable. The question still remained as to what measurement of head size correlates with variation in P100 latency and how this variable should be used in clinical studies. The present study of 22 normal volunteers demonstrates that the transverse (preauricular notch to preauricular notch) head measurement is a major determinant of P100 latency. Gender and other head measurements were not found to make a statistical difference. Use of head-size-normalized data for clinical studies should improve both the sensitivity and specificity of the PVEP.