Background Informal care makes an important contribution to societal welfare. However, it may involve substantial time costs and can have a considerable negative effect on the health and well-being of informal caregivers. These costs and effects of informal caregiving are often excluded in economic evaluations of healthcare interventions. The impact of this exclusion on the outcomes of these evaluations is largely unknown. Objectives This study aimed to explore the inclusion of informal care in economic evaluations and the potential impact of the costs and effects of informal caregiving on cost-effectiveness outcomes. Methods A systematic review was conducted to identify economic evaluations of interventions in four distinct disease areas where informal care is potentially important: Alzheimer’s disease, metastatic colorectal cancer, Parkinson’s disease and rheumatoid arthritis. It was recorded how often economic evaluations included informal caregiving. Next, for the studies including informal care, the impact on cost-effectiveness outcomes was determined by removing informal care costs and effects of the cost-effectiveness calculations and recalculating the outcomes. The new cost-effectiveness outcomes were then compared with the original reported outcomes. Results The study identified 100 economic evaluations investigating interventions targeted at Alzheimer’s disease (n = 25), metastatic colorectal cancer (n = 24), Parkinson’s disease (n = 8) and rheumatoid arthritis (n = 43). Twenty-three of these evaluations (23 %) included costs and/or effects of informal caregiving: 64 % of the Alzheimer’s disease studies, 0 % of the metastatic colorectal cancer studies, 13 % of Parkinson’s disease studies and 14 % of rheumatoid arthritis studies. When informal care was included, this mostly concerned time costs. Studies rarely included both costs and effects. The effect of including or excluding informal care costs or effects on cost-effectiveness outcomes in most studies was modest, but in some studies the impact was strong. Conclusion Most economic evaluations in the area of Alzheimer’s disease include costs and/or effects related to informal caregiving. However, in other disease areas where informal caregiving is common it seems that the majority of economic evaluations ignore informal caregiving. The inclusion of informal care can have a strong impact on cost-effectiveness outcomes. Future economic evaluations should therefore consider the relevance of informal care in the context of their study, and either include these costs and effects or justify why they were excluded.