Mitotic figures were observed in 38 cases of a series of 3200 CSF cytograms. Their relative incidence was the highest in cases of tumoural invasion of the CSF, but they also occurred in inflammatory and reactive fluids and did not necessarily indicate tumoural origin. In bacterial infections of the central nervous system they were mainly present during the repair phase of the disease. In haemorrhagic spinal fluid they were absent. These findings suggest that cell division in spinal fluid tends to develop in conditions in which the viability of the cells is not affected by introduction of foreign material such as blood and bacteria.