Introduction Clinic absenteeism promotes higher waiting lists for medical procedures and public resources waste. Objectives The present work aimed to identify the reasons for clinic absenteeism from each cycle of the antineoplastic chemotherapy treatment, as well as to determine the socio-demographic, clinical and treatment profiles of this population. Methods This observational prospective work evaluated pediatric and adult patients which missed their chemotherapy cycle between May and October 2023 in a Cancer Center located in Rio de Janeiro, Brazil. Clinic absenteeism rate was calculated, and socio-demographic profile was described. Reasons for absenteeism, treatment protocol and most used drugs were also identified. Results This work analyzed data from 69 patients, the majority above 60 years old. Approximately 60% were male, 33.3% had little to no education and 63.8% lived outside the center city. Absenteeism average monthly rate was 1.73% for adults and 0.87% for children. The most related non-attendance reasons were patient feeling too ill to attend their chemotherapy session, failure to remember the cycle day and lack of means of transportation. Most prevalent neoplasms were from the digestive tract (46%). Fluorouracil, irinotecan, oxaliplatin and gemcitabine were the most discarded drugs due to absenteeism. Conclusions Older patients and the ones residing far away from the Center tend to miss the scheduled chemotherapy cycles. However, most reasons for absenteeism could be avoided by confirmation calls or text messages. These procedures implementation could lead to a lower absenteeism rate and less resource waste.