Variation in language on the basis of formality (register variation) is often neglected both in grammatical descriptions and in sociolinguistic analyses. I demonstrate here that in Sinhala, and perhaps in other diglossic languages, register variation in syntax cannot be ignored. In a Head-Driven Phrase Structure Grammar (HPSG) analysis based on a corpus of naturally occurring Sinhala texts, I propose an analysis of register variation in which the syntax of all observed registers is accounted for within a single grammar. I further explain how the approach to register variation developed here can be extended to other types of sociolinguistic variation.