An extensive literature review has been conducted to observe the statistical correlation of the plasticity index, PI, with the liquid limit, LL, of smectite and kaolinite. Fifty-nine data for smectite and fifty-one for kaolinite have been plotted and compared to each other. The results show that PI is related to LL with eqs. PI = 0.97 × LL − 37.6 for smectite and PI = 5.94e^0.023·LL for kaolinite. An independent data set was used for the validation of the proposed relationships. Besides, it was possible to identify a confidence interval for PI, relative to a certain interval for LL values, to confirm the robustness of the relations given above. The findings of this research show that the relation between the Atterberg limits is clearly controlled by the clay mineralogy and that there is no unique way to get PI from LL if the clay mineralogy is not considered.