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An investigation of a Serratia marcescens outbreak in a pediatric cardiac intensive care unit (CICU) suggested that under-staffing or overcrowding might have been underlying risk factors.To assess the effect of fluctuations in CICU nurse staffing levels and patient census on CICU nosocomial infection rate (NIR).The monthly CICU nursing hours, patient days and nosocomial infections were obtained from retrospective review of administrative, patient and microbiology records during December, 1994, through December, 1995 (study period). The NIR and nursing hours:patient day ratio were then calculated. The correlations between NIR vs. nursing hours, patient days and nursing hours:patient day ratio were determined.The median monthly CICU NIR was 6.9 (range, 0 to 15.2) infections per 1000 patient days; the median number of hours worked per month by CICU registered nurses was 7754 (range, 7133 to 8452) hours; the median number of patient days treated per month was 507 (range, 381 to 590) patient days; and the median monthly nursing hours:patient day ratio was 15.2:1 (range, 13.2:1 to 19.9:1). The strongest linear correlation was observed between the monthly NIR and patient days (r = 0.89, P = 0.0001). There was an inverse correlation between the monthly NIR and nursing hours:patient day ratio (r = −0.77, P = 0.003).The NIR was most strongly correlated with patient census but also was strongly associated with the nursing hours:patient day ratio. These factors may influence the infection rate because of breaks in health care worker aseptic technique or decreased hand washing. Increased patient census alone may increase the risk of cross-transmission of nosocomial infections. As hospitals proceed with cost containment efforts the effect of fluctuations in patient census and nurse staffing on patient outcomes needs evaluation.