BACKGROUND: Health care workers (HCWs) use their mobile phones during working hours or medical care. There is evidence that the instruments are colonized with pathogenic microorganisms. Here, we describe levels of Enterobacteriaceae contamination (EC) in cell phones and the risk factors associated with EC in Peruvian intensive care units (ICUs). METHODS: This was a 5-month cohort study among 114 HCWs of 3 pediatric and 2 neonatology ICUs from 3 Peruvian hospitals. A baseline survey collected data on risk factors associated with EC. Swabs were collected from HCWs' phones every other week. RESULTS: Three-quarters of HCWs never decontaminated their phones, and 47% reported using the phones in the ICU >5 times while working. EC was frequent across samplings and sites and was substantially higher in subjects with longer follow-up. Potential risk factors identified did not have strong associations with positive samples (relative risk, 0.7-1.5), regardless of significance. Half of the phones were colonized with an Enterobacteriaceae at least once during the 4 samplings attained on average during the study period. Half of the isolates were multidrug resistant (MDR), and 33% were extended-spectrum beta-lactamase producers. CONCLUSIONS: EC on HCWs' phones was frequent and apparently randomly distributed through the hospitals without clear clustering or strongly associated risk factors for having a positive sample. Based on the level of EC, phones may be considered as potential bacterial reservoirs of MDR and ESBL bacteria.