Gathering information from parents on developmental history and on actual functioning is an important element of diagnosis. Usually clinicians take it for granted that the data provided this way are reliable. We used two types of questionnaires, one containing open-ended questions, the other consisting of closed questions. By comparing the answers to both types filled in by fathers (2 x 50) and mothers (2 x 50) independently we tried to verify the interreliability of the data collected. We found a general degree of unanimity of 82%, dropping to only 48% on questions regarding early milestones of speech and language development. We found differences between the two types of questionnaires: more questions were left unanswered in the open-question type questionnaire, more differing answers were observed in a closed-question type questionnaire. Some factors might have had an influence on reliability, such as socio-economic background of the parents, differences between data on first and only children versus children from larger families. Although data collecting, including history taking, is, and will stay an important source for information gathering, clinicians have to be careful in interpreting them.