We report two cases of sirenomelia, a rare congenital defect with a prevalence rate of 1:100 000 births; both cases were observed in Cali, Colombia. Both pregnant women were referred from Buenaventura, Colombia. The expecting mothers shared multiple adverse sociodemographic factors. Their homes were located in a city where the entire population is of low socioeconomic status living under conditions of extreme poverty. They were uneducated, with nutritional deficiencies and no access to drinking water most of the time. Both were exposed to water and fish from a nearby river contaminated with leachate from a poorly managed landfill. A similar relation was previously reported in Cali in 2005 between environmental factors and sirenomelia. We suggest that there is a common aetiological factor of environmental origin between these two sirenomelia cases and propose that exposure to derivatives from landfills should be included among the factors for this rare defect of multifactorial aetiological origin.