The paediatric examination with immunization is a stressful event for toddlers. Its characteristics are close to the classic ''Strange Situation'': a perceived threat, which activates the attachment system, a stranger, and an unfamiliar environment. As such, it can be used as a natural situation to assess the quality of the mother-child attachment, on one hand to manage the stress of the toddler during the paediatric examination, and on the other to screen for relationship disturbances. To this end, we have developed a specific coding instrument, the Paediatric Attachment Style Indicator (PASI), which allows classification of infant and toddler behaviour in the paediatric examination as secure, anxious-avoidant, or anxious-ambivalent. This article presents the PASI instrument and its validation. Paediatric examinations (N ¼ 41 mother and toddler dyads) were videotaped during a booster session of the regular immunization schedule for toddlers in Switzerland. Results of the PASI coding were compared with other methods of direct observation of emotional behaviour of the toddler (distress signals) and of the mother (verbal behaviours), with the information given by the mother to the toddler before the examination, and with the assessment of temperament of the toddler by the paediatrician. Results show good face validity and construct validity for the instrument.