Originally established as a football club, San Isidro has gained recognition for its rugby union team, holding a record of 33 Torneo de la URBA championships.

[3] At the beginning of 1902, a group of young men started to practise football in a field placed in the lowest part of the hill (called "la barranca" in Spanish).

Nevertheless, the frequent swells of the Río de la Plata interrupted the matches many times, causing the boys to stop playing the sport they loved.

That group of boys was encouraged by some respectable neighbours of the zone, such as Pedro Becco or Manuel Aguirre, and even Avelino Rolón (who was Intendent of San Isidro Partido in 1906).

As a result, soon after Aguirre led part of the lands for an indefinite period of time, where "Club de Foot-ball San Isidro" was founded.

By that time (May 1902) a group (formed by English natives and sons of British), most of them employees of the Central Argentine Railway also began to play football in a field next to the railroad station.

San Isidro played all the Primera División championships from 1906[6] to 1931,[7] disaffiliating from AFA when football became professional in Argentina (although the squad remained one season in the amateur league before the two associations merged in 1934).

[9][10][11] Since then, football has been practised at CASI –at amateur level– to present, with senior, youth, children and women squads competing in the tournaments organised by the club.

The first rugby team of San Isidro was formed in 1908, combining British-origin employees of the Central Argentine Railway and a few Argentine-born, but it was dissolved in 1911 without having participated in any competition.

[14] San Isidro has won 33 URBA titles and one Nacional de Clubes to date, becoming the most winning rugby union club of Argentina.

In 1917 CA San Isidro affiliated to The River Plate Rugby Union, playing the RPRU championship (and winning their first title) still wearing the red shirt.