Enlarging upon the scope and agenda of earlier diagnostic research, a study was conducted to examine (1) teacher responses to receiving diagnostic information about some of their students, (2) effects on student achievement of teachers receiving both diagnostic information and diagnostic training, (3) diagnostic classifications that emerged from student performance, and (4) differential teacher effectiveness and instructional practices. Subjects included 10 experienced fifth-grade classroom teachers. A reading diagnostic battery, developed for the previous studies, was individually administered as a pretest to 186 of the 192 students in the teachers' classrooms. An experimental group of five teachers, randomly assigned, received four hours of training in diagnostic reliability. Before and after the training, all 10 teachers diagnosed one of four randomly assigned simulated cases of reading difficulty at two different times. Teachers were also interviewed three times. Results showed that teachers responded positively to the receipt of specific diagnostic information about their students, seeing it as useful for thinking about and making changes in their instructional practices. Students for whom teachers received diagnostic information did not show significantly improved achievement over students for whom no information was provided. (Appendixes include the reading diagnostic battery and the interview questions.) (HOD)