Chemotherapy-associated adverse events (AEs) including neutropenia, anemia, thrombocytopenia, mucositis and dehydration are common and often occur together in clusters. As a consequence, it makes little clinical sense to discuss the costs associated with an isolated AE, ignoring real-life clustering. This study estimates the costs of AEs based on a new approach-the clinically relevant AE cluster. The study population included all cancer patients receiving chemotherapy (N = 2,067) over 1 year at The University of Pennsylvania Health System. AE clusters were determined using algorithms based on the start and end dates of AEs and dates of chemotherapy administration. Models were developed to estimate the total and specific daily costs associated with the presence of AE clusters using loglinear variance estimation. The study found that the cost of a hospitalized day was $1,444 (95% CI 1,340-1,548). Neutropenia in isolation added another $130 (-278-538) to a hospitalized day while neutropenia plus thrombocytopenia increased daily costs by $461 (244-677) and the cluster of anemia, neutropenia and thrombocytopenia added $840 per day (629-1,051). Anemia alone added $568 (346-789) to the hospital day and mucositis, in conjunction with other AEs, increased the daily hospital costs by $466 (201-732). Estimating the costs of chemotherapy-associated AEs can be achieved using clinically meaningful chemotherapy-associated AE clusters. Models incorporating clusters can be utilized to estimate the expected costs of chemotherapy associated AEs.