The objective of the presented research was to test whether working memory, measured using the Spin the Pots task, is an important factor in passive vocabulary development in 2- and 3-year-old children. Two longitudinal studies were conducted. In the first, 135 children participated in the first study. At 18 months their responding to joint attention was measured, and then at 24 months their working memory and passive vocabulary was tested. It was demonstrated that responding to joint attention predicts the level of development of working memory, which in turn influences the extent of the passive vocabulary. In the second study, 113 children participated - at 30 months their working memory was measured, and at 24 months and then 36 months their passive vocabulary was tested. It was observed that at the age of 3 working memory is still a significant predictor of passive vocabulary. The usefulness of the Spin the Pots task for measuring the working memory of young children was shown and discussed in conclusions.