OBJECTIVES: To identify generalizable ways that comorbidity affects older adults' experiences in a health service program directed toward an index condition and to develop a framework to assist clinicians in approaching comorbidity in the design, delivery, and evaluation of such interventions. DESIGN: A qualitative data content analysis of interview transcripts to identify themes related to comorbidity. SETTING: An outpatient low-vision rehabilitation program for macular disease. PARTICIPANTS: In 2007/08, 98 individuals undergoing low-vision rehabilitation and their companions provided 624 semistructured interviews that elicited perceptions about barriers and facilitators of successful program participation. RESULTS: The interviews revealed five broad themes about comorbidity: (i) "good days, bad days," reflecting participants' fluctuating health status during the program because of concurrent medical problems; (ii) "communication barriers." which were sometimes due to participant impairments and sometimes situational; (iii) "overwhelmed," which encompassed pragmatic and emotional concerns of participants and caregivers; (iv) "delays," which referred to the tendency of comorbidities to delay progress in the program and to confer added inconvenience during lengthy appointments; and (v) value of companion involvement in overcoming some barriers imposed by comorbid conditions. CONCLUSION: This study provides a taxonomy and conceptual framework for understanding consequences of comorbidity in the experience of individuals receiving a health service. If confirmed in individuals receiving interventions for other index diseases, the framework suggests actionable items to improve care and facilitate research involving older adults.