We examined the effects of a professional development (PD) system designed to support teachers' use of data-based instruction (DBI) to improve early writing outcomes for children with intensive needs. The PD system, called "DBI-TLC," provided tools for early writing assessment and intervention, learning modules including face-to-face workshops followed by classroom application, and ongoing coaching to support DBI implementation. Special education teachers in 19 classrooms in 2 Midwestern districts were assigned randomly to receive DBI-TLC or to a business-as-usual control group. All teachers completed pre- and posttests of DBI knowledge and skills and self-efficacy, and DBI-TLC teachers' fidelity to DBI was assessed. Fifty-three students (2 to 3 from each classroom) completed pre- and posttests of early writing using curriculum-based measures (CBM) and the Test of Early Written Language-3 (TEWL-3). DBI-TLC teachers outperformed controls at posttest on DBI knowledge and skills (Hedge's g = 2.88) and reported a more explicit writing instruction orientation compared to controls (g = 1.63). DBI fidelity varied (on average, 84% for assessment, 79% for intervention, and 52% for decision-making). Students whose teachers implemented DBI showed a pattern of stronger early writing performance compared to control students on CBM, with effect sizes of 0.23 to 0.40, but not on the TEWL-3 (0.02 to 0.13). We discuss the promise of DBI-TLC to improve teacher practice and student outcomes, as well as the need to continue to explore ways to support teachers' implementation of DBI with fidelity. [For the corresponding grantee submission, see ED595445.]