CALL glossing in first language (L1) or second language (L2) texts has been shown by previous studies to be more effective than traditional, paper-and-pen L1 glossing. Using a pool of studies with much more statistical power and more accurate results, this meta-analysis demonstrates more precisely the degree to which CALL L1 glossing can be more effective than traditional L1 glossing. Results indicate, as previous research has shown, that CALL L1 glossing is significantly more effective in L2 reading comprehension than traditional L1 glossing. That is, the mean effect size is significantly higher (p less than 0.001) for studies that use CALL L1 glosses (g = 1.44) when compared to studies that use paper-based L1 glosses (g = 0.50). This article explains how and under what circumstances CALL L1 glosses may be more or less effective than traditional L1 glosses. (Contains 5 tables and 3 notes.)