Illness cognitions are mediators between illness and well-being in patients with physical symptoms. The Illness Cognitions Questionnaire (ICQ) explores these illness cognitions but has not been validated in older persons. This study aimed to validate the ICQ in adults aged 60 years and above and to develop an extended version (ICQ-Plus) suitable for older persons. Qualitative interviews were conducted to explore illness cognitions in 21 older persons suffering physical symptoms. The items in the original ICQ and items of these interviews that potentially reflect dimensions not covered by the original ICQ were combined in the ICQ-Plus. Then the ICQ-Plus was completed by 220 older patients with physical symptoms, and its factor structure was explored by maximum-likelihood factor analyses. Analysis of covariance was performed to assess differences in illness cognitions between older persons suffering from medically explained symptoms (MES) and medically unexplained symptoms (MUS). The interviews generated 26 new items. Factor analysis confirmed the 3-factor structure of the original ICQ, including factors covering helplessness, disease benefits, and acceptance. In addition, exploratory factor analysis on the ICQ-Plus items revealed 4 additional factors, including cognitions referring to perseverance, illness anxiety, avoidance, and catastrophizing. Patients with MUS scored significantly lower than did patients with MES on acceptance and disease benefits and higher on helplessness and illness anxiety. We concluded that older patients with physical symptoms express illness cognitions that are relevant for treatment but are not covered by the ICQ and recommend that the extended ICQ-Plus be used in studies of older persons. (PsycINFO Database Record