The aim of this study was to clarify a possible relationship between pollution and worsening of headache in the industrial city of Turin. From October 1992 to June 1993, we examined a group of 32 patients suffering from various headache types. During these months, they kept a daily record of their headaches and associated disturbances. Changes in pain frequency and severity were recorded every hour of the day and compared hour to hour with the various degrees of pollution recorded in the main streets by a monitoring station. The influence of meteorological parameters was also taken into consideration. During winter, carbon monoxide and nitrogen dioxide showed a simultaneous hyperconcentration on the same days and the same hours. Increased incidence of headache attacks and increase in severity corresponded to the same hours, days, and months. The findings were statistically significant (P = 0.008, Student's t-test). An isolated increase in nitrogen dioxide only (without an increase in carbon monoxide which was only recorded once) induced headache a couple of hours after the peak concentration was reached. Among the meteorological factors, only the highest values in wind velocity were shown to exert a significant influence on worsening headache frequency and severity.