The article analyses the change in the Estonian conjunction kuna 'while; as, because' from temporal to causal one as well as the attitude of the Estonian language planners towards this change. This change has mainly taken place during the twentieth century. The end of the nineteenth century and the beginning of the twentieth century witnessed the prevailing use of the temporal kuna. However, during the 20th century the use of causal kuna began to dominate. The impact of this shift in usage of kuna as a causal conjunction has also slightly changed the use of other Estonian causal conjunctions. Initially, Estonian language planners adopted a strict attitude towards the change of kuna, but language planning could not halt this change. Editors continued to cross out the causal use of kuna, but they could not increase the temporal use, and this usage of conjunction hardly appears in the written text of 1970s.However, the change was finally accepted in 1990s.