Background: Preservatives such as formaldehyde and formaldehyde-releasers are common causes of contact allergy. Objectives: To examine trends in contact allergy to formaldehyde and formaldehyde-releasers in patch tested patients in Denmark over a 10-year period (2007-2016), and to investigate relevant sources of formaldehyde among the patients. Methods: A cross-sectional registry study on patch test data from patients tested with formaldehyde and formaldehyde-releasers (N = 8463) was performed. The presence of released formaldehyde in products from formaldehyde-allergic patients was identified with chemical analyses (chromotropic acid or acetylacetone test). Results: The prevalence of contact allergy to formaldehyde 1% was 1.5%, and ranged between 0.97% and 2.3%, with a decreasing trend in this 10-year period. Contact allergy to formaldehyde 2% was found in 2.4%, and no significant trend was observed. Quaternium-15 was the formaldehyde-releaser most often positive (0.86%). Patients allergic to formaldehyde often had simultaneous positive patch test reactions to formaldehyde-releasers (36%). Almost 63% of the patients with formaldehyde allergy used products that released formaldehyde; cosmetics were the most common sources. Conclusions: Although contact allergy to formaldehyde 1% decreased in this 10-year time period, contact allergies to formaldehyde and formaldehyde-releasers overall remain frequent in patients. In most cases, formaldehyde-allergic patients are exposed to ≥1 products containing formaldehyde. Improved regulation on permitted amounts of free formaldehyde in cosmetics is still warranted, including direct labelling of formaldehyde when it is present in small but relevant amounts.