The paper serves as a resource for researchers interested in building the knowledgebase on MCC. Current MCC research has focused primarily on studying the impact of high-prevalence diseases (i.e. hypertension, hyperlipidemia, diabetes, arthritis, etc.) in terms of patient outcomes, care utilization and cost. However, an understudied group comprises patients with less prevalent combinations of MCC. How the group may change over time as individuals acquire new chronic conditions, or certain conditions change in intensity, has not been well examined. There are many unique constellations of MCC; for example, a recent study of approximately 32 million Medicare beneficiaries found over 2,000,000 unique combinations of MCC (Sorace et al. 2011). The distribution of constellations of MCC results in a curve with a very “long tail” of complex patients that changes nationally over time. Sources and methods for studying the long tail and recommendations for future research on less prevalent MCC are the primary focus of our paper.