The relationship of mean velocity of optokinetic nystagmus (OKN) to pursuit eye movements (PEM), to vestibular nystagmus and to voluntary saccades was analysed in 10 patients with peripheral vestibular lesions and in 30 patients with central vestibular lesions. PEM and vestibular nystagmus were significantly correlated to OKN, suggesting that a common neural pathway is used in the generation of these eye movements. Weak or no correlation was found between saccadic peak velocity and slow phase velocity of OKN. Using multiple linear regression analysis, it was found that 78.5% of the variation in the slow-phase velocity of OKN could be explained by a synthesis of PEM and vestibular test data. PEM test data were more powerful than those of vestibular nystagmus in deduction of OKN. The possible appearance of slow build-up of OKN could not be deduced from the reduction of PEM. Hence, the relationship between PEM and OKN in man is not a simple linear one, but is more complex.