This study set out to inspect Laufer and Hulstijn's (2001) motivational-cognitive construct of task-induced involvement. Involvement load hypothesis predicts that tasks with the same involvement loads lead to the same amount of learning. To this end, three output task types which induce equal involvement indices of 3 were employed to examine the extent to which they promoted learners' vocabulary acquisition and retention. Participants were 78 Iranian EFL learners at the intermediate level from three intact classes who were assigned to one of the three experimental groups. Participants in the first group were required to make sentences using the target words; and participants in the second group were required to summarize the texts using the target words; while participants in the third group were required to predict the end of the two incomplete stories using the target words. The results revealed that the three tasks significantly affected vocabulary learning. However, it was found that the three tasks with the same loads of involvement led to significant differences in the learning and retention of the target words. It was concluded that when task-effectiveness is concerned, involvement load is not the only determining factor.