RATIONALE & OBJECTIVE: Valid measures of side effects are important to inform clinical use of calcineurin inhibitors (CNIs). This study developed and established the content validity of a patient-reported outcome (PRO) measure to capture side effects among kidney transplant recipients taking CNIs. STUDY DESIGN: Qualitative interviews for concept elicitation and cognitive debriefing. SETTING & PARTICIPANTS: Participants enrolled in the qualitative study were adults who had received a kidney transplant≤5 years earlier and were currently taking a CNI for immunosuppression. ANALYTICAL APPROACH: Concept elicitation interview data were analyzed by developing a codebook from a list of symptoms described by participants that they attributed to CNIs or their immunosuppression regimen and by applying the constant comparative approach separately by 2 coders. The most bothersome and important side effects reported by participants were used to select existing PRO items. Cognitive debriefing examined item comprehensibility, comprehensiveness, and relevance. RESULTS: Among 24 participating patients, 12 underwent concept elicitation interviews. Most participants (n=10) reported neurological or cognitive CNI side effects, and a majority of those (n=6) reported the neurological and cognitive side effects were the most bothersome adverse consequences of therapy. Based on these findings, 16 items were either selected from the Functional Assessment for Chronic Illness Therapy (FACIT) PRO item library or were newly written. These items were refined and reduced to 13 for cognitive debriefing, which was completed by 18 participants. These items were organized into 2 scales: Tremors and Cognitive Side Effects. LIMITATIONS: Though a diverse set of patients participated, they were not representative of all recipients of kidney transplants. The PRO measure's reliability, convergent and known-groups validity, and responsiveness to change were not evaluated. Symptoms could not be definitively attributed to CNIs. CONCLUSIONS: A newly developed PRO measure, the Functional Assessment of Chronic Illness Therapy-Calcineurin Inhibitor-Neurotoxicity (FACIT-CNI-Ntx), captures important side effects among patients receiving CNIs. PLAIN-LANGUAGE SUMMARY: People undergoing kidney transplantation require a lifelong regimen of immunosuppressing medications, including calcineurin inhibitors. Calcineurin inhibitors have side effects that may negatively impact transplant recipients' lives, sometimes interfering with important daily activities. We created a new survey for kidney transplant recipients to report on their experiences with calcineurin inhibitor side effects that we named the Functional Assessment of Chronic Illness Therapy-Calcineurin Inhibitor-Neurotoxicity (FACIT-CNI-Ntx) and that captures important side effects among patients receiving CNIs.