The current study was an attempt to investigate the possible effects of lexical inference strategy training on Iranian upper-intermediate EFL learners' vocabulary depth and breadth. This quasi-experimental study was conducted among 45 upper-intermediate EFL learners at Simin Language Institute, in Rasht-Iran, who were selected through convenience sampling. All learners were female, aged 18 to 40, from three intact classes who were randomly assigned to one experimental group and one control group. In the experimental group, the participants were taught how to infer the meaning of new words from the context, employing lexical inference strategies. The control group received the traditional method of teaching vocabulary. The results of the t-test analysis indicated that the participants in the experimental group who received instruction on lexical inference strategies outperformed their counterparts in the control group with regard to their vocabulary depth and breadth. The findings may have implications for EFL teachers, language learners, and syllabus designers.