• Parental persistence in the diagnosis and treatment of a child's illness can be examined by asking whether it is congruent with the child's morbidity. The relationship between parental persistence and child morbidity always depends on a "first diagnosis" (ie, what is wrong with the child, if anything?). In many instances, a "second diagnosis" (ie, why is the child being brought to you at this time?) should not be overlooked. First and second diagnoses are identical if persistence and morbidity are congruent but are different if they are incongruent. Once incongruent persistence is suspected, the possibility of parental falsification of symptoms must be faced. In patients in whom a second diagnosis must be made, the second diagnosis may be as simple as maternal anxiety or as complex as Munchausen syndrome by proxy. A "persistence algorithm" is presented and illustrated by descriptions of four children referred to a pulmonologist with complaints suggestive of respiratory disease. The approach, as outlined in the algorithm, may have broad pediatric applications.