OBJECTIVES: Cancer patients receiving chemotherapy may often require chemotherapy dose delays and dose reductions due to chemotherapy-induced side effects such as thrombocytopenia, neutropenia, and anemia. In responsive cancers, such dose delay and dose reduction of chemotherapy may be associated with decreased effectiveness and poorer clinical outcomes. In this study, we analyzed the characteristics of dose delay among cancer patients on chemotherapy. METHODS: Data from 2000–2007 from a large electronic database of outpatient oncology clinics in the US were analyzed. Adult patients with a cancer diagnosis and complete chemotherapy plan information were eligible for this analysis. Chemotherapies were grouped into platinum-, anthracycline-, gemcitabine-, taxane-based, and other. The planned cycle duration was compared to the actual duration. A difference of more than 7 days was defined as a cycle delay. RESULTS: The dataset included 47,159 cancer patients (mean age 61 (SD 13), male 42%) and 75,243 chemotherapy regimens. A total of 26,317 patients (113,175 chemo cycles) had complete chemotherapy plan information. Of these, 5961 (22.7%) experienced chemo delays in 9293 cycles (8%) during chemotherapy. The average length of delay was 17 days (SD, 8 days; median, 14 days); 25% of the delays were more than 3 weeks. Tumor types of the 5,961 patients with cycle delays included, colorectal cancer (20.3%), NSCLC (18.6%), breast cancer (15.2%), ovarian (4.4%), and H&N (2.9%). The reasons for delaying chemo were not specified in the database. However, many (60%) experienced anemia (Hb<11g/dL) or thrombocytopenia (<150,000µl, 62%) at some point during chemotherapy. CONCLUSIONS: Chemotherapy cycle delays were relatively common in this large group of cancer patients. While decisions to delay therapy were multi-factorial, further research is needed to investigate the reasons, preventable measures, clinical and cost outcomes of these delays.