Introduction Schistosomiasis is still a major public health problem in Brazil, where it is assumed that there 2 500 000 to 8 000 000 cases of the disease, despite the development of control activities. The North-eastern region, where is situated the state of Maranhão, is the most affected area. This study analyses trends of the percentage of positive cases of Schistosomiasis in Maranhao, between 1997 and 2003.

Methods We analysed a time series of percentages of positive cases, grouped according to 15 Regional Health Authorities in the state, available in the information system of the Schistosomiasis Control Program (SISPCE). Trend analysis was performed through polynomial regression models. Statistical significance, coefficients of determination (R²) and residuals were used to define the best model.

Results The percentage of positive cases of schistosomiasis amounted to 7.42% in the study period. Two Regional Health Authorities showed constant downward trends with an average annual percentage of positive cases of 1.72 and 0.66, respectively. Meanwhile, the others showed upward trends.

Conclusions The declining trends suggest the possibility of underreporting. It is believed that data registration in the Regional Health Authorities has been affected by changes in the activities of the SISPCE, due to decentralisation of the Program to municipalities. In some cases, best living conditions could have contributed to the observed declines. However, deficit of environmental sanitation and poverty are significant in the majority of the state and certainly are related to the increments observed in the other regionals.