To study the spread of methicillin-resistant Staphylococcus aureus (MRSA) in hospitals, especially secondary MRSA infections in relation to the origin of the MRSA strain and the measures taken regarding contact isolation. Secondary data analysis. As part of the national MRSA surveillance of the National Institute of Public Health and the Environment, data were collected using questionnaires. The analysis covered the period July 1994-June 1996 and was performed for index cases of secondary infections versus sporadic cases. Possible risk factors were determined. In the study period 30 index cases of (clusters of) secondary infections and 191 sporadic cases were found. The size of the clusters was limited. Strict contact isolation as described in the guidelines of the Dutch Working Group on Infection Prevention prevented secondary infections in most cases. Patients for whom no relation could be found between the MRSA infection and a stay abroad were found to have caused more secondary infections, even when the data were corrected for contact isolation measures.