There is a growing interest in incorporating informal care in cost-of-illness studies as a relevant part of the economic impact of some diseases. The aim of this paper was to review the recent literature valuating the costs of informal care in a group of selected diseases from 2005 to 2015. We carried out a systematic review on the economic impact of informal care, focusing on six selected diseases: arthritis or osteoarthritis, cancer, dementia, mental diseases, multiple sclerosis and stroke. We selected 91 cost-of-illness articles. The average weight attributed to the informal care cost over the total cost was highly relevant for dementia, stroke, mental diseases, cancer and multiple sclerosis. The most frequent valuation method applied was the opportunity cost method, followed by the proxy good method. The annual cost of informal care presented a high variability depending on the disease and geographic location. Distinguishing by type of illness, the disease with the highest annual value of informal caregiving was dementia, followed by mental illness and multiple sclerosis. The average hourly unit cost was €11.43 (2015 values), varying noticeably depending on the geographic location. This paper identifies several aspects that should be enhanced to promote comparability between studies and countries, and it sends key messages for incorporating informal care costs to adequately measure the economic impact of diseases.