PIP: An intense process of epidemiological transition due to the change in mortality has superseded the eras of pestilence, pandemic disease, and chronic-degenerative diseases in Mexico. But the change has not been uniformly progressive, for instance, recurrence of malaria after its eradication indicates reverse transition. Enteritis, influenza, and pneumonia ranked among the primary causes of death between 1955 and 1957, whereas by 1980 heart disease became the leading cause followed by accidents, influenza, and enteritis. A polarized epidemiology can be observed in the post-transitional period: the poorest segment of the population continues to have high rates of infections and nutritional deficiencies. According to Mexican health service mortality data gathered in 1981 industrial workers had similar rates of infectious and chronic-degenerative diseases (11.7% and 12.0%, respectively), while agricultural workers had high rates of infectious and parasitic diseases (26.7%) and a low rate of chronic-degenerative diseases (5.6%). Illnesses of perinatal origin were also higher in agricultural workers than in industrial workers (26.1% vs. 13.7%), whereas circulatory diseases were more frequent among industrial workers (14.2% vs. 7.2%). It is the task of the health care system to meet the challenge of the complexity of this epidemiological mosaic.