During vesical carcinogenesis a variety of genetic alterations such as oncogene mutation or loss of suppressor genes have been detected. Codon 12 mutation of the c-K-ras gene has been seen with a high frequency in several human neoplasias but its participation in the development of vesical cancers has not been fully dilucidated. Using the DNA restriction fragments polymorphism (RFLP) technique enhanced by a polymerase chain reaction (PCR) a study has been made of codon 12 mutation at the c-K-ras gene in 55 patient with vesical cancer undergoing surgery between 1991 and 1992. The tumoral stage was superficial (Ta-Tl) in 24 cases, infiltrant (T2-T4) in 28 cases and unknown in 3 cases. Two patients (3.6%) showed codon 12 mutation at the c-K-ras gene. One case was a fast evolving infiltrant tumour (T2-T3) which caused death of the patient after 4 months while the other case was a surface tumour (G2Ta) which relapsed early, the pathological anatomy revealing a stage T2-T3 squamous carcinoma. Our results suggest that codon 12 mutation at the c-K-ras gene is not a meaningful genetic change in the genesis of vesical cancer. Its emergence, however, appears to be related to a more aggressive tumoural behaviour.