OBJECTIVE To assess the differences in the prevalence of muscarinic and nicotinic type symptoms and the level of erythrocytic cholinesterase, prior to pesticide exposure, in male and female migrant agricultural workers. METHODS A cross-sectional study was carried out in 2001 in Sinaloa State, Mexico, among 488 migrant workers. A questionnaire was applied and erythrocytic cholinesterase levels were measured before the beginning of the agricultural season. The differences by sex were compared using the "t" test for continuous variables and chi2 test for categorical variables. Prevalence odds ratios were also estimated. Statistical significance was assessed using p-values <0.05 and 95% confidence intervals. RESULTS Significant differences were found by age, migration type, place of origin, education, and migration time (p=0.000). Women were six times more likely to have anemia and asthma, twice more likely to have parasites and respiratory and gastrointestinal diseases and 38% more likely to suffer from heart disease. They were also at greater prevalence in thirteen of nineteen investigated symptoms. The average cholinesterase level was within normal limits (4.22 U/ml+/-0.77) and it was similar to the levels reported using the Magnotti method. CONCLUSIONS The prevalence of symptoms, illnesses and cholinesterase levels found in this study may serve as baseline values for future comparisons of the health effects of pesticide exposure. The English version of this paper is available at: http://www.insp.mx/salud/index.html.