Lateralization, the change of a coronal nasal into a lateral in an l-n sequence, has been considered to be prosodically unrestricted, e.g. an utterance-span rule, in Korean (Han 1993, Park 1990). However, aerodynamic data of the nasal do not corroborate their claims. In the paper, I look at how lateralization can best be characterized. Specifically, I ask whether its domain is best treated via a syntax-based (Nespor & Vogel 1986, Selkirk 1984) or an intonation-based approach (Pierrehumbert 1980, Jun 1993) to prosodic structure. Based on nasal airflow data as a means of monitoring velum activity coincident with a nasal stop in an l-n sequence, combined with pitch tracks to define an accentual phrase, I argue that lateralization is neither an utterance-span rule nor a syntax-based rule. Sentences recorded with a potential environment for lateralization show that lateralization occurs within an accentural phrase but is blocked between accentual phrase boundaries. When intonation-based and syntax-based models disagree about phrase boundaries, lateralization only occurs where the intonation-based model predicts it will. This indicates that lateralization is best defined as an accentual pheonomenon, being sensitive to the accentual phrase. This finding lends further support to an intonation-based model for Korean prosodic structure (Jun 1993).