Background/Aims: Tattooing is a global trend. Clinical knowledge of complications is based on case reports collected over a century. Larger cohorts reflecting complications associated with contemporary trends are lacking. Methods: The study was a retrospective review of a consecutive cohort of patients with tattoo complications diagnosed in the “Tattoo Clinic“ of Bispebjerg University Hospital in Copenhagen, Denmark, from 2008 to 2015, based on patient history and systematic clinical examination. Results: A total of 493 tattoo complications in 405 patients were studied. Overall, 184 (37%) presented allergic reactions with plaque elevation in 32.2%, excessive hyperkeratosis in 3.7%, and ulceration in 1.4%, predominantly observed in red tattoos and nuances of red; 66 (13%) presented papulo-nodular reactions, mainly observed in black tattoos (considered non-allergic) and due to pigment agglomeration; 53 (11%) had bacterial infections; 46 (9%) were psycho-social complications; 144 (30%) belonged to several specific diagnostic entities, including photosensitivity, pain syndrome, and lymphopathy. We found no cases of cutaneous or other malignancies. Sarcoidosis was primarily seen in black tattoos and was a common associated disease, found in 23 reactions (5%), compared to the background population. Conclusion: The study introduces a new concept of classification of tattoo complications based on simple tools such as patient history and objective findings supplemented with histology. The study reflects complications originating from presently used tattoo inks, often with organic pigments. The introduced classification has been submitted to the World Health Organisation (WHO) as a proposal to the 11th revision of the International Classification of Diseases.