OBJECTIVES: Poorly controlled CINV may lead to additional office or emergency room visits, thus, increasing the overall costs of cancer care. This study estimates the societal costs of uncontrolled CINV for working-age cancer patients. METHODS: Employees or spouse and/or dependents with cancer who received highly or moderately emetogenic chemotherapy were identified from a 1997-2002 proprietary dataset linking medical claims to work-loss information. Patients were followed from the earliest date of chemotherapy for up to six-months, excluding those with less than three-months of continuous enrollment. Direct medical costs were measured using payments, normalized as monthly, and updated to 2004 USD. Work loss days were identified from employment records. Costs of uncontrolled CINV were derived by comparing medical costs and work-loss days for three groups, patients with uncontrolled CINV and no ER visit, with uncontrolled CINV and ER visit, and with controlled CINV, using the Wilcoxon Mann-Whitney test in univariate analyses. All patients with uncontrolled CINV were pooled as one group in multivariate analysis. RESULTS: In all, 2,071 patients were identified; 25% required medical care for uncontrolled CINV; 2% had ER visits. Compared with patients with controlled CINV ($8,132), total direct costs were significantly higher for patients with uncontrolled CINV, no ER ($10,376, P<0.001) and uncontrolled CINV and ER ($12,810, P<0.001), respectively. Estimated work-loss days were 6.1, 7.2, and 8.9 days the above groups, respectively. After controlling for demographics, geographic regions, and comorbidities, the difference in monthly medical costs between the controlled and uncontrolled group was $2619 (P<0.001). However, the difference in work-loss (0.21 days) was no longer significant (P=0.73). CONCLUSIONS: Uncontrolled CINV was associated with a significant increase in medical costs. For patients with uncontrolled CINV but no ER visit, increases in cost were driven by outpatient care, whereas for those with ER visits, inpatient care was the major cost driver.