We compared the specialty listings of physicians in the Yellow Pages of the 1983 Hartford, Connecticut, telephone book with the board certifications in specialties of the American Board of Medical Specialties as listed in the American Medical Association directory or the Marquis Directory of Medical Specialists. There were 1179 listings by 946 physicians under 61 specialty headings in the Yellow Pages. We found that a mean of 12 percent of "specialists" listed in the Yellow Pages were not board-certified in a specialty, although they had had ample opportunity to obtain board certification. We conclude that specialty advertising in the Yellow Pages is potentially misleading to consumers and that member boards of the American Board of Medical Specialties should consider ways to diminish this possible misrepresentation.