Background.Extended-spectrumβ-lactamase (ESBL)–producing strains of bacteria have become a major public health concern. In the present study, the incidence of carriage of ESBL-producing strains was analyzed for general trends and seasonality.Methods.Monthly data on ESBL-producing strains were collected retrospectively at 2 large university hospitals in Germany. The mean monthly temperatures for the 2 settings were collected from Germany's national meteorological service. Multivariable time series analyses were performed to explain variations in the monthly incidence densities of carriage of ESBL-producing bacteria (number of cases involving ESBL-producingEscherichia coliand/orKlebsiellaspecies per 1,000 patient days). For the final models, we incorporated variables for the ascending linear trends and other variables representing the mean monthly temperature.Results.Our models demonstrated that there was an increasing trend in the incidences of carriage of ESBL-producing bacteria. In addition, the incidences of carriage of all ESBL-producing bacteria responded positively to the mean temperature, meaning that during the summer, more cases involving ESBL-producing bacteria were detected than during the winter. The same methodology was also applied to the incidence of methicillin-resistantStaphylococcus aureuscarriage, but no association was found with the mean temperature.Conclusions.In the present study, we demonstrated that the monthly incidence of carriage of ESBL-producing bacteria was highly correlated with the mean monthly temperature, a fact that should be considered in experimental studies as an additional parameter influencing the incidence of ESBL-producing bacteria.