A meta-analysis of 29 separate studies investigating pretest effects was conducted. Outcomes of the studies (achievement gains or attitude improvements) were computed as standardized differences between pretested and non-pretested groups. Eleven other variables were coded for each outcome. Initial descriptive statistics were indicative of differences between randomized and nonrandomized studies, so all further analyses were based on the 110 randomized group outcomes. For all outcomes, the average effect size was .34, indicating the general elevating effect of pretest on posttest. Cognitive outcomes were raised .51, attitude outcomes .33, and others about .27 standard deviations. Seventy percent of all effects were positive, and 93% of the cognitive effects were positive. Duration of time between pre- and post-testing was not related to effect size, although effect size is generally smaller for durations over one month. Year of publication, sample size, presence of experimental treatment, and sameness or difference of pretest and posttest were not significantly related to effect size. Researchers must continue to include pretest as a design variable when it is present, and to estimate its effect. (Author/CP)