A longitudinal study investigated the effects of leisure time reading (reading at home for pleasure or fun) on pupils' reading achievement in school. Subjects, 736 students in grades 3, 4, 5, and 6 in 30 schools located throughout the Netherlands, had their reading achievement determined five times: at the beginning and end of grade 3 and at the end of grades 4, 5, and 6. The frequency of reading in leisure time was measured intensively with diaries in grades 3, 4, 5, and 6 during 8, 12, 6, and 10 weeks, respectively. Variables such as reading attitude, reading aloud, television viewing, and the reading time at school were controlled. Contrary to widely held assumptions, results indicated no support for the notion that leisure time reading was an important factor in the development of reading proficiency at school. Findings suggest that the initial premise must be reformulated. Leisure time reading may have an effect on the reading proficiency of students only when the amount of reading, the quality of the reading process, and the quality of books being read reach certain threshold values. (Contains 26 references, 4 notes, 2 tables of data, and 5 models.) (RS)