Active surveillance techniques using routine telephone contacts with providers improved the reporting of measles, rubella, salmonellosis, and hepatitis by a factor of 4.6 among private physicians in Monroe County, New York, and increased reporting for these target diseases from all sources by 51 percent. The timeliness of reporting was not improved by active surveillance. Reporting patterns varied by disease and source of report, suggesting the desirability of various approaches to surveillance based on local resources and priorities. Although reporting rates were higher for diseases among persons from census tracts of low socioeconomic status, physicians providing care to persons living in low-income areas responded no differently to active reporting than did those providing care to patients from middle- and high-income areas.