This paper will evaluate the effect of the open primary elections system, in the three countries -Argentina, Chile and Uruguay- in which this institutional engineering instrument was formally adopted and regulated by the electoral organism of the country. We will try to find out if the open primary procedure is necessary (if the benefits sought by its application emerge as responses to existing problems in society) and whether it is appropriate (if certain conditions exist that allow such benefits to outweigh the hidden costs that it entails), evaluating its functioning in every election since its incorporation, and comparing the results to ascertain the evolution of the implementation of this norm. We note that these advantages have not been obtained so far, in any of the three cases (particularly in Argentina, where the party system is much weaker and less institutionalized than in those of its counterparts). The use of this system resulted in an improvement of the political parties in none of these cases, giving rise instead to the establishment of a type of nonpartisan, personalized or divided competition in sectors. This ended up promoting fragmentation, internal inconsistency and the loss of relevance of political parties within the framework of a democratic system.En este trabajo se indaga acerca del sistema de internas o primarias abiertas, en los tres países –Argentina, Chile y Uruguay- en los que éstas fueron formalmente adoptadas, a través de una perspectiva metodológica comparada. Aquí procuraremos averiguar si el procedimiento de las primarias abiertas es necesario (es decir, si los beneficios perseguidos con su aplicación emergen como respuestas a problemáticas existentes en la sociedad) y si es conveniente (si se producen determinadas condiciones que permitan que tales beneficios superen los costos ocultos que acarrea), evaluando todas las elecciones transcurridas desde su incorporación. Observamos que los beneficios buscados no se han obtenido hasta ahora, en ninguno de los tres casos; lo cual terminó promoviendo la fragmentación, la inconsistencia interna y la pérdida de relevancia de los partidos políticos en el marco de un sistema democrático.