Introduction: Methemoglobinemia is a disease of great importance due to its non-specific symptoms and the potentially fatal consequences. However, in Brazil, its prevalence has not yet been detected in primary care, making public health strategies for identification and early diagnosis impossible. Objective: evaluate the prevalence and factors related to methemoglobinemia, to use a pulse COoximeter in patients of Basic Health Units in the city of Anápolis-GO. Methodology: 482 patients, over 18 years old, who were undergoing outpatient consultations in primary care in the city of Anápolis-GO, between August and October 2016 were included. Users answer a questionnaire on socio-demographic data and measures of carboxyhemoglobin, methemoglobin and oxyhemoglobin. Results: Using a definition of meta-hemoglobinemia when a meta-hemoglobin (MetHbA)> 1%, the prevalence of meta-hemoglobinemia in primary care patients was 16%. There was a negative correlation between oxygen saturation and methaemoglobin concentration (coefficient:-0.30; p <0.0001) and a positive correlation between carbon monoxide concentration and methaemoglobin concentration (coefficient: 0.14 ; p = 0.002). A multivariate logistic regression showing that oxygen saturation, OR: 0.49; 95% CI 0.40-, 60; p <0.0001 and the concentration of carbon monoxide OR: 1.27; 95% CI 1.13-1.43; p = 0.001 predict the presence of methaemoglobinaemia. Conclusion: A minimum prevalence (16%) of methemoglobinemia was identified in patients with primary care. These are factors related to the presence of methemoglobinemia, reduced oxygen saturation and increased concentration of carbon monoxide.