Formaldehyde and formaldehyde-releasers are common causes of allergic contact dermatitis. To determine the frequency of sensitization to formaldehyde and seven formaldehyde-releasers. To establish and characterize groups of patients according to the results of patch testing. We performed a 5-year retrospective study, in six Spanish hospitals, of patients with positive patch test reactions to formaldehyde or any of seven formaldehyde-releasers. The most frequent allergens were formaldehyde (1.72%), imidazolidinyl urea (1.05%), quaternium-15 (0.88%), and diazolidinyl urea (0.79%). Patients with sensitization to only formaldehyde had a higher frequency of occupational dermatitis (25%) than patients with sensitization to only formaldehyde-releasers (9.5%). The most common sites of dermatitis were the hands (31.7%) in patients with sensitization to only formaldehyde and the face and legs (31.3% and 24.6%) in patients with sensitization to only formaldehyde-releasers. We found a subgroup of 25 patients who were sensitized to both imidazolidinyl urea and diazolidinyl urea, and only 6 of these (24%) were also sensitized to formaldehyde. The inclusion of imidazolidinyl urea and diazolidinyl urea in the baseline series of the Spanish Contact Dermatitis and Skin Allergy Research Group (GEIDAC) should enable better classification of patients allergic to formaldehyde, and could aid in their management.