What passes for diagnostic and prognostic uncertainty in communication between doctor and patient is not always or wholly a function of medicine's level of scientific and clinical knowledge. This became evident in a study of what parents of children with poliomyelitis were and were not told by the hospital's treatment staff concerning the amount of residual handicapping to be expected. While the staff became increasingly certain in their prognosis, the families were allowed to remain optimistically uncertain for a long time. In medical practice the communication of uncertainty or certainty can take many guises, and these, it is suggested, will vary, depending in large part on the organizational context of the doctor-patient relationship.