Seclusion of psychiatric inpatients is used as a last resort in managing disturbed behaviour. Research on factors contributing to its use is limited. The aim of this study was to examine the effect of a change in unit size and other variables, such as time of day and nursing workload, on the rates of use of seclusion in an intensive care inpatient psychiatric unit. Rates of seclusion were examined in randomly selected nursing shifts across three time periods; one before and two after a change in the size of the unit. Effects on seclusion rates of size, shift and four nursing workforce variables were explored using multivariate analysis. Unit size, shift and nurse hours together explained 23% of the variance in seclusion use. Of particular significance was the change from a 20 bed unit to two separate 10 bed units. The findings have implications for the organisation of inpatient psychiatric units. Smaller units may allow better management of disturbed behaviour.