Background: Multidrug chemotherapy (CT) is still associated with relevant side-effects. We assessed, under current practice patterns, frequency and severity of CT-induced toxicity and its economic consequences.Patients and methods: Prospective, multicentre, longitudinal, observational cohort study with lymphoproliferative disorder (LPD) and non-small-cell lung cancer (NSCLC) patients, receiving first-or second-line (immuno-) CT (excluding myeloablative CT). Data were collected from patient interviews and preplanned chart reviews. Costs in 2007 euros are presented from the provider perspective.Results: Two hundred and seventy-three patients (n = 153 LPD; n = 120 NSCLC) undergoing a total of 1004 CT cycles were assessable (age ‡65 years, 40%; female, 36%; Eastern Cooperative Oncology Group performance status ‡2, 11%; tumour stage ‡III, 56%; history of comorbidity, 80%). Fifty percent of cycles were associated with grade 3/4 toxicity and 37% (n = 371) with at least one hospital stay (outpatient/day care n = 154; intensive care n = 19). Mean (median) toxicity-related costs amounted to €1032 (€86) per cycle. Costs rose exponentially with the number of grade 3/4 adverse drug reactions (ADRs) and were highest in cycles affected by more than four ADRs, €10 881 (€5455); in cycles with intensive care, €14 121 (€8833); and in cycles affected by grade 3/4 infections and febrile neutropenia/ leukopenia, €7093 (€4531) and €5170 (€2899), respectively. Five percent of CT cycles accounted for 56% of total expenses.Conclusions: Individualised supportive care strategies are needed. Future research should focus on identifying toxicity clusters and patient characteristics predictive for high costs.