This longitudinal study examined the effects of a comprehensive professional development program on literacy environments of preschool classroom/teacher units. The Early Language & Literacy Classroom Observation Toolkit (ELLCO) was used to assess effects of treatment. Forty (40) classroom/teacher units, representing 14 preschool centers, participated in the study (treatment group n = 20; control group n = 20). The treatment group of classroom/teacher units received mentoring by early childhood specialists, professional development training, and literacy materials. The control group of classroom/teacher units received literacy materials. Both the treatment and control groups of classroom/teacher units received Read Together, Talk Together kits along with training in use of the kits. The ELLCO measure was administered twice each year in treatment and control classroom/teacher units by qualified teams of assessors, via a subcontract arrangement with a university. Educational researchers served as external evaluators for the study. The data were analyzed using multivariate repeated measures of analyses of variance. Statistically significant (p less than 0.001) effects were found for Group, Time, and the Group by Time interactions. All associated multivariate partial eta-squares exceeded 0.60, indicating strong effects. Also, follow-up, univariate analyses of variance indicated that all Group, Time, and Group by Time interactions were statistically significant (p less than 0.001), with associated univariate partial eta-squares ranging from 0.26 to 0.72. Group means across the six measurements indicated little or no gain in classroom literacy environment means for the control group of classroom/teacher units, but substantial gains were made in the means for the treatment group of classroom/teacher units over the same period of time. (Contains 6 tables.)