The aim of the study was to investigate how the input modality affects the processing of a morphologically complex word. The processing of Finnish inflected vs. monomorphemic words and pseudowords was examined during a lexical decision task, using behavioral responses and event-related potentials. The stimuli were presented in two modalities, visually and auditorily, to two groups of participants. Half of the words and pseudowords carried a case-inflection. At the behavioral level, the inflected words elicited a processing cost with longer decision latencies and higher error rates. At the neural level, pseudowords elicited an N400 effect, which was more pronounced in the visual modality. Inflected words elicited an N400 effect in both modalities, which, however, differed in topography and latency. The N400 effect for inflected words most probably reflects access and possible integration of the stem and suffix. The results suggest that the inflectional processing cost stems from the later, lexical-semantic stage of processing in both modalities. The ERP responses to inflected pseudowords did not differ from the ERP responses to monomorphemic pseudowords in either modality, suggesting that combinatorial case-inflection processing requires a real word stem in order to proceed.