BACKGROUND: Studies conducted in transitional communities from Africa and Asia have pointed to the process of urbanisation as being responsible for the increase in asthma prevalence in developing regions. In Latin America, there are few published data available on the potential impact of urbanisation on asthma prevalence. The aim of the present study was to explore how the process of urbanisation may explain differences in asthma prevalence in transitional communities in north-eastern Ecuador. METHODOLOGY/PRINCIPAL FINDINGS: An ecological study was conducted in 59 communities in Esmeraldas Province, Ecuador. Indicators of urbanisation were grouped into three indices representing the processes associated with urbanisation: socioeconomic, lifestyle and urban infrastructure. Categorical principal components analysis was used to generate scores for each index and a fourth index--a summary urbanisation index--was derived from the most representative variables in each of the three indices. The authors analysed the associations between community asthma prevalence and the indices, as well as with each indicator variable of every group. The overall prevalence of asthma was 10.1% (range 0-31.4% between communities). Three of the four indices presented significant associations with community asthma prevalence: socioeconomic (r = 0.295, p = 0.023), lifestyle (r = 0.342, p = 0.008) and summary urbanisation index (r = 0.355, p = 0.006). Variables reflecting better socioeconomic status and a more urban lifestyle were associated with greater asthma prevalence. CONCLUSIONS: These data provide evidence that the prevalence of asthma increases with increasing levels of urbanisation in transitional communities, and factors associated with greater socioeconomic level and changes towards a more urban lifestyle may be particularly important.