OBJECTIVES: The probability of contracting a hospital acquired infection increases the longer an individual is hospitalised.  The purpose of this analysis is to quantify the additional probability of contracting Clostridium Difficile Infection (CDI) for each additional day spent in the hospital. METHODS: Using the English Hospital Episode Statistics (HES) database, the number of inpatient days before diagnosis of hospital-acquired CDI and the total number of inpatient days for patients that do not acquire CDI are calculated. This time-to-event variable, along with a set of covariates, is used in a binary logistic regression to predict the added probability of contracting CDI for each additional day in the hospital.   Patients in this analysis were over 50 years old, had a length of stay (LoS) less than 50 days, and had a co-morbidity of chronic obstructive pulmonary disease (COPD), heart failure, diabetes, or kidney disease RESULTS: The average LoS for all patients, patients with CDI, and patients without CDI, was 5.48 days (σ2=8.46), 26.24 days (σ2=12.86), and 5.38 days (σ2=8.46), respectively. The average LoS before acquiring CDI was 6.27 days (σ2=12.81).  For each additional day spent in the hospital, a patient has 4.40% greater chance of contracting the disease (OR=1.044; s.e.=0.001; p<0.01).  Increased age (OR=1.134; s.e.=0.019; P<0.01) and being female (OR=1.28; s.e.=0.034; p<0.01) were also predictors of CDI diagnosis.  CONCLUSIONS: This model quantifies the increased likelihood of hospital acquired infections associated with longer duration inpatient admissions.  Such infections are associated with much more severe health outcomes and are a significant burden on hospital resources.