Mathematics standards expect students to communicate about mathematics using oral and written methods, and some high-stakes assessments ask students to answer mathematics questions by writing. Assumptions about mathematics communication via writing include (a) students possess writing skill, (b) students can transfer this writing skill to mathematics writing, and (c) mathematics writing is representative of a mathematics knowledge. We conducted a study in which we investigated the connections among general writing ability, mathematics computation skill, and mathematics writing. With 155 fourth-grade students in two regions of the United States, we administered a measure of essay writing, a measure of mathematics computation skill, and two mathematics-writing prompts. Results indicate moderate correlations among general writing ability, computation skill, and mathematics writing. Additionally, general writing ability and computation skill are significantly related to mathematics-writing outcomes.