Leprosy is an important public health problem in many developing countries and many features of its determinants are still obscure. To investigate whether the incidence of leprosy is related to certain environmental and socioeconomic determinants, an ecological study was undertaken in 165 municipalities of the state of Ceará, Brazil. Social, economic, education, sanitation, demography, meteorology, and health data were collected. The dependent variable was the average incidence rate of leprosy from 1991 to 1999. Simple and multiple linear regressions were performed to assess the relationship between the dependent and the independent variables. The average incidence rate for all the municipalities for the 1991-1999 period, varied from 0.06 to 14.68 per 10000 persons per year. The level of inequality (beta = 1.67, P = 0.011), the mean years of study among the population >or=25 years old (beta = 1.35, P < 0.001), the population growth from 1991 to 1996 (beta = 0.02, P = 0.007), the percentage of children 7-14 years old that did not go to the school (beta = 0.02, P = 0.028), and the presence of a railroad in the municipality (beta = 0.45, P = 0.038) were found to be predictors of the incidence rate of leprosy in Ceará. Our findings fit the assumption that, in Ceará, leprosy is associated with a high level of poverty and uncontrolled urbanization. We put forward the hypothesis that urbanization increases not only social inequality eventually leading to strong polarization, but also excludes people from social and material opportunities. Apparently, such deprivations render them susceptible for leprosy.