Purpose Cancer treatment is increasingly delivered in an outpatient setting. This may entail a considerable economic burden for family members and friends who support patients/survivors. We estimated financial and time costs associated with informal care for colorectal cancer. Methods Two hundred twenty-eight carers of colorectal cancer survivors diagnosed on October 2007–September 2009 were sent a questionnaire. Informal care costs included hospital- and domestic-based foregone caregiver time, travel expenses and out-of-pocket (OOP) costs during two phases: diagnosis and treatment and ongoing care (previous 30 days). Multiple regression was used to determine cost predictors. Results One hundred fifty-four completed questionnaires were received (response rate = 68 %). In the diagnosis and treatment phase, weekly informal care costs per person were: hospital-based costs, incurred by 99 % of carers, mean = €393 (interquartile range (IQR), €131–€541); domestic-based time costs, incurred by 85 %, mean = €609 (IQR, €170–€976); and domestic-based OOP costs, incurred by 68 %, mean = €69 (IQR, €0–€110). Ongoing costs included domestic-based time costs incurred by 66 % (mean = €66; IQR, €0–€594) and domestic-based OOP costs incurred by 52 % (mean = €52; IQR, €0–€64). The approximate average first year informal care cost was €29,842, of which 85 % was time costs, 13 % OOP costs and 2 % travel costs. Significant cost predictors included carer age, disease stage, and survivor age. Conclusion Informal caregiving associated with colorectal cancer entails considerable time and OOP costs. This burden is largely unrecognised by policymakers, service providers and society in general. These types of studies may facilitate health decision-makers in better assessing the consequences of changes in cancer care organisation and delivery.