Voluntary laboratory reporting of communicable disease, including bacteraemia, has been the mainstay of surveillance in England for >30 years. The impact of introducing a parallel mandatory reporting process for surveillance of Staphylococcus aureus bacteraemia [both methicillin susceptible (MSSA) and resistant (MRSA)] was assessed by national and regional comparison of MSSA and MRSA reports to the two surveillance systems. Introduction of mandatory reporting in 2001 demonstrated that the true number of cases was 40% higher than indicated by voluntary reporting (i.e. 60% case ascertainment by voluntary reporting). However by 2008 the difference in reporting of MRSA bacteraemia between the two systems dropped to 30%, with six of the nine health regions in England having improved their levels of voluntary reporting, although there was still under-reporting from London, the South East and the North West. Improvements in voluntary surveillance contributed to increased ascertainment of bacteraemia due to S. aureus (both MRSA and MSSA). Decreasing trends for MRSA bacteraemia were evident in both surveillance systems, with a 56% decrease in the mandatory and a 53% decline in the voluntary systems, from 2004. In contrast there was little change in reported cases of MSSA during 2004-2006. However, in 2007, when MRSA bacteraemia case numbers decreased by 27%, MSSA bacteraemia case reports actually increased by 6%. Trends for MSSA bacteraemia can be assessed more accurately from voluntary than from mandatory surveillance at the present time because mandatory reporting of MSSA bacteraemia is incomplete, with only 133 of 170 (78%) Trusts reporting in all four quarters of a year.