The role of environment in the spread of nosocomial infection has been acknowledged. One way to control the spread of infection is to control and monitor patient environments to prevent transmission. Studies applying the suggested aerobic colony count standards to monitor environmental contamination were undertaken over an 18-month period at both a London pediatric hospital and in adult intensive care units. The resulting data demonstrate that a large proportion of sites screened for bacterial contamination would fail if using the criteria suggested by previous authors-particularly those sites closest to patients-suggesting a new standard might be required.