Purpose: Longitudinal integrated clerkships (LICs) are innovative educational models that emphasize medical student continuity with patients, preceptors, peers, and health systems. We characterize LIC growth in the US and interpret the growth using Rogers’ Diffusion of Innovation Theory. Methods: In 2015, we surveyed 123 US allopathic medical schools affiliated with Clerkship Directors in Internal Medicine (CDIM). The organization’s annual survey was supplemented with questions aimed to quantify the number of current and planned LICs and to determine the intended purpose of starting LICs. Results: Of the 94 (out of 123 possible) schools which were responding, 35 (37%) have at least one LIC of six months or greater; of these 20 are year-long. Nineteen schools are engaged in planning a new LIC or increasing the number of students in an LIC. At least 45 (48%) responding schools will have LICs in future years. Respondents report implementing LICs to foster continuity of care, support patient-centeredness, advance inter-professional education, and address workforce shortages. Conclusions: The number of LICs is increasing across the US. We considered the data through the lens of Diffusion of Innovation Theory, speculated that LIC growth has reached “critical mass,” and considered why the LIC innovation may be self-sustaining.