BACKGROUND In recent years, a steep increase in the frequency of occupational contact allergy to isothiazolinones has been reported from several European countries. OBJECTIVE To examine the extent and occurrence of isothiazolinones in different types of product at Danish workplaces. METHODS Seven different isothiazolinones were identified in the Dictionary of Contact Allergens: Chemical Structures, Sources, and References from Kanerva's Occupational Dermatitis. By use of the chemical names and Chemical Abstracts Service numbers for these chemicals, information on products registered in the Danish Product Register Database (PROBAS) was obtained. RESULTS All seven isothiazolinones were registered in PROBAS. The top three isothiazolinones registered were: benzisothiazolinone (BIT), registered in 985 products, methylisothiazolinone (MI), registered in 884 products, and methylchloroisothiazolinone (MCI)/MI, registered in 611 products. The concentration ranges were 0.01 ppm to 45% for BIT, 0.01 ppm to 10% for MI, and 0.01 ppm to 14.1% for MCI/MI. The most common product type was 'paint and varnish'; five of the seven isothiazolinones were registered in this type of product. CONCLUSION Isothiazolinones are present in multiple products registered for use at workplaces, and may occur in high concentrations.