Genetic improvement of the maize cultivars IAC-1There are presented results of the improvement of the maize (Zea mays, L.) cultivars IAC-1, after the introduction of prolificacy in a population formerly improved for eight cycles by the modified ear-to-row procedure. It was obtained an average progress of 3.3% per cycle in grain yield, against an expected genetic progress of 7.2% per cycle. For the ear index a genetic progress of 8.9% per cycle was observed and an expected genetic progress of 1.9% per cycle was estimated. With the introduction of prolificacy factors, ear index increased considerably outyielding the commercial double hybrid control Lodging changed 'very little with the cycles of selection. The IAC-1 cultivars showed an accumulated progress of 28% for grain yield and 10% for ear index. The gain in ear index is mainly due to the prolific inbred lines introduced in the synthetic.