Language acquisition crucially depends on the ability of the child to segment the incoming speech stream. Behavioral evidence supports the hypothesis that infants are sensitive to the rhythmic properties of the language input. We recorded event-related potentials (ERPs) to varying stress patterns of two syllable items in adults as well as in 4- and 5-month-old infants using a mismatch negativity (MMN) paradigm. Adult controls displayed a typical MMN to the trochaic item (stress on the first syllable) as well as to the iambic (stress on the second syllable) item. At the age of 4 months, no reliable discrimination response was seen. However, at the age of 5 months, a significant mismatch response (MMR) was observed for the trochaic item, indicating that the trochee, i.e. the most common stress pattern in German, was separated consistently from the iambic item. Hence, the present data demonstrate a clear development between 4 and 5 months with respect to the processing of different stress patterns relevant for word recognition. Moreover, possible contributions of different filter settings to the morphology of the mismatch response in infants are discussed.