The present study investigated the effects of auditory selective attention on the processing of syntactic information in music and speech using event-related potentials. Spoken sentences or musical chord sequences were either presented in isolation, or simultaneously. When presented simultaneously, participants had to focus their attention either on speech, or on music. Final words of sentences and final harmonies of chord sequences were syntactically either correct or incorrect. Irregular chords elicited an early right anterior negativity (ERAN), whose amplitude was decreased when music was simultaneously presented with speech, compared to when only music was presented. However, the amplitude of the ERAN-like waveform elicited when music was ignored did not differ from the conditions in which participants attended the chord sequences. Irregular sentences elicited an early left anterior negativity (ELAN), regardless of whether speech was presented in isolation, was attended, or was to be ignored. These findings suggest that the neural mechanisms underlying the processing of syntactic structure of music and speech operate partially automatically, and, in the case of music, are influenced by different attentional conditions. Moreover, the ERAN was slightly reduced when irregular sentences were presented, but only when music was ignored. Therefore, these findings provide no clear support for an interaction of neural resources for syntactic processing already at these early stages.