Acquisition of methicillin resistant Staphylococcus aureus (MRSA) in the intensive care unit of a tertiary referral centre was monitored over a 19-month period. The incidence of new cases of MRSA correlated with peaks of nursing staff workload and times of reduced nurse/patient ratios within the unit. This implies that nurse understaffing contributes significantly to the spread of MRSA in an ITU setting.