This paper examines the causes of nonsteady-state behaviour in manual assembly lines and shows that several of them will result in transient behaviour which is equivalent to line ‘start-up’ conditions where interstage buffer stores are either all empty or all full of part finished items. These two conditions of & lsquo;empty start’ and & lsquo;full start’ are simulated in order to determine the magnitude of the initial deviation of operating behaviour from steady-state and the length of time required for the line to resume steady-state behaviour. Expressions are developed which relate transient magnitude and length to the number of stations in the line and the interstation buffer capacity. Lines whose mean station work times have been imbalanced are also simulated under similar nonsteady-state conditions. It is concluded that in certain conditions the performance of some lines subject to repeated disturbance can be improved by deliberate station work time unbalancing.