Objective: To systematically review, synthesize, and appraise available evidence, connecting adult ADHD (aADHD) with somatic disease. Method: Embase, Psychinfo, and Medline databases were searched for studies published from 1994 to 2015 addressing aADHD and somatic comorbidity. Somatic conditions were classified according to International Classification of Diseases (ICD-10) codes. Levels of evidence were graded as inconclusive, tentative, or well documented. Results: Most of the 126 studies included in the qualitative synthesis were small and of modest quality. Obesity, sleep disorders, and asthma were well-documented comorbidities in aADHD. Tentative evidence was found for an association between aADHD and migraine and celiac disease. In a large health registry study, cardiovascular disease was not associated with aADHD. Conclusion: There are few large systematic studies using standardized diagnostic criteria evaluating aADHD and somatic comorbidities. Significant associations are found between aADHD and several somatic diseases, and these are important to consider when assessing and treating either aADHD or the somatic diseases.