The origin and breeding of the São Paulo cotton variety IAC 12 as well as the involved proceedings and numerical results are reported. A program of crosses between strains of several Upland varieties was iniciated in 1942/43 which ended in 1959 when the first bags of foundation seeds of IAC 12 were delivered for final increase. IAC 12 variety originated from a cross between strains of the varieties Stoneville 2B and Delfos followed by a backcross to the later and five cycles of selection and progeny tests. This new variety not only solved the problem of lint length that depressed cotton culture of the State of São Paulo around the year of 1956, but also improved the general aspect of plants and most of the agronomic and technological characters.