Between 1985 and 1988, a two-phase epidemiological study was conducted at the "Ruta 100" company in Mexico City to examine the harmful effects of the working conditions on drivers of urban transport vehicles. In the first phase, five groups of nine drivers each were interviewed regarding their opinions about their working conditions and possible work-related disorders. On the basis of this information, an epidemiological survey was developed and applied using a polytypic sampling design to some 200 drivers plus two other groups of approximately the same size, one consisting of maintenance workers and the other of office workers, which served as controls. The driver group showed a high prevalence of respiratory and neuropsychiatric disorders, diarrhea, myopia, arterial hypertension, urinary disorders, hemorrhoids, hearing loss, back pain, varicose veins, peptic ulcers, diabetes, cardiopathies, abdominal hernias, and appendicitis. For arterial hypertension, hemorrhoids, nervous disorders, and psychiatric disorders the prevalence ratios of drivers/maintenance workers and drivers/administrative workers were statistically significant.