General practitioners and psychiatrists communicate mainly by letter. To ascertain the most important items of information that should be included in these letters ("key items") questionnaires were sent to 80 general practitioners and 80 psychiatrists. A total of 120 referral letters sent to psychiatric clinics in 1973 and 1983 were studied, together with the psychiatrists' replies, and these were rated for the inclusion of "key items." General practitioners' letters contain less information about the family but more about psychiatric history than they did a decade ago. Overall, psychiatrists' letters have not changed. Registrars, however, now include noticeably more "key items" than they did 10 years ago, but their letters remain twice the length of those written by consultants. It is suggested that letter writing skills are vital to good patient management and should be taught to postgraduate trainees in general practice and psychiatry.