To describe the role of health care providers in the delayed diagnosis of pulmonary tuberculosis (PTB) among the adult population in a city in Colombia.A case study of PTB diagnosis among adults in the city of Medellín, Colombia, in 2007. Quantitative and qualitative data were collected through in-depth interviews with 19 patients (11 with early diagnosis and 8 with late diagnosis), from medical records, and by a survey of PTB-related knowledge and practices among 89 doctors and nurses at health care clinics. A research workshop was held with patients to share the results and validate new categories.A lack of PTB knowledge and patient stereotyping were found to lead to misinterpretation of symptoms and the use of symptomatic treatments. The lack of knowledge regarding risk factors and clinical protocols made diagnosis difficult when the patient did not match the stereotype or presented with a more well-known illness. Misinformation on PTB transmission and inappropriate measures led to discriminatory practices and social isolation and only reinforced stereotypes and stigma, which in turn discouraged the quest for medical attention.Health care providers played a large part in the late diagnosis of PTB in the adults studied. Beliefs regarding social stigma and a lack of knowledge about the disease-shared by providers and patients-contributed to stereotyping of the illness and the disease-carrier, and limited the probability of an early diagnosis of PTB.