Purpose: Infusion filters, 0.2 μm, are commonly used in intensive care units as in-line filters to minimize particle and microbiological burden on patients. These filters usually contain either a positively charged or an uncharged membrane. The aim of the present study was to identify and to quantify an additive causing an unexpected maximum at 234.5 nm in the ultraviolet spectrum of a filtered drug solution using a filter. Materials and Methods: For identification and quantification of the substance, water for injection was used as eluent. Measurements were done by mass spectrometry and ultraviolet spectroscopy. Results: The unexpected additive in the filter was found to be N,N-dimethylacrylamide. The eluate of 2 filter batches had a mean N,N-dimethylacrylamide concentration of 3.9 and 2.5 μg/mL, respectively. Further investigations showed that after infusion breaks of different duration, the N,N-dimethylacrylamide concentration at the beginning of the next infusion cycle reaches a higher level than that at the end of the preceding infusion cycle. Conclusions: The type of filter examined in this study is primarily used in premature infants, newborns, and infants. These patients are vulnerable to neurotoxic substances having a long-term effect. Therefore, N,N-dimethylacrylamide should be completely removed from the final filter product.