The relationships among input uncertainty, means of coordination, and criteria of the organizational effectiveness of hospital emergency units were explored using data from 30 emergency units in six midwestern states. Input uncertainty generally was not associated with the use of various means of coordination. However, input uncertainty affected relationships between the means of coordination and the effectiveness criteria. Specifically, programmed means of coordination made a greater contribution to organizational effectiveness under conditions of low uncertainty than under conditions of high uncertainty. Conversely, nonprogrammed means of coordination made a greater contribution to organizational effectiveness when uncertainty was high than when it was low. Findings were interpreted and suggestions were advanced as to how emergency units might best solve their coordination problems under varying conditions of uncertainty.