Slaughterhouse workers are at high risk of accidents and pathogens contamination. Besides the high technology and large production, the slaughter industry still has clandestine settlements in several cities of Brazil's countryside. This illegal activity represents severe risks to the health of consumers and slaughterhouse workers. Only a few studies show the health of clandestine slaughterhouse workers, considered as a hidden population because of their illegal activity. Thirty workers (6 women and 24 men) answered a questionnaire about the work and health conditions of clandestine slaughtering from a small city in the Northeast of Brazil. We also did local observations of the slaughtering, which increased the panoramic view of this activity. Most slaughterhouse clandestine workers are young, have a little educational background, and work without legal rights. Clandestine workers - men and women - have severe health problems related to the activity of cattle slaughtering, which includes physical injuries, metabolic diseases, and psychic disorders. Women appear to be more affected by these health problems. Working conditions are adverse, dangerous, and offer risks of contamination, injuries, and chronic diseases. The social structure of Brazil's countryside lacks legal support for slaughterhouse workers, which causes them poor health and suffering. The workers' poorness, low income, and low education level can explain the vulnerability to the diseases observed in illegal slaughtering. This is the first study to analyze the health of illegal slaughterhouse workers in Brazil's countryside.