A synthesis of findings from 38 studies on group-based applications of mastery learning strategies are presented. Meta-analytic procedures were used to combine the results of the studies and to calculate overall estimates of the effects of group-based applications. Results show that such applications yield consistently positive effects on both cognitive and affective student learning outcomes, and several teacher variables. However, variation in the size of the effect across studies is quite large. Effect size was found to vary as a function of the grade level of students, the subject area to which mastery learning is applied, and the duration of the study. Possible explanations for this variation are discussed, along with implications for future directions in the research. Tables, figures, and a 10-page reference list are appended. (Author/PN)