Diazolidinyl urea (DU; trade name Germall II), a new broad spectrum preservative, was tested from July 1984 to September 1988 in 2400 consecutive patients with eczemas possibly caused or complicated by a contact allergy. In addition to the European standard series (Trolab), a supplementary test battery including DU 2% in water was used. A sensitization to DU was seen in 13 patients (0.54%). Of 13 patients 7 reacted only to DU, but not to formaldehyde (FA) or FA releasers; in these patients, the reaction can be attributed to sensitivity to DU itself. Six of the 13 patients reacted to DU as well as FA and all FA releasers tested; in these patients, the reaction to DU is probably due to released FA. For practical reasons, DU should be added to the list of FA releasers. In some of the patients, concomitant sensitization to frequent contact allergens such as nickel, cobalt, chromate, or a long history of skin disease could be found; it seems reasonable to assume that in these cases the sensitization to DU was enhanced by an eczematous skin with a reduced barrier function. Patients reacting to FA should be advised to perform a repeated open application test (ROAT) before using a cosmetic containing DU as a preservative, to prevent possible adverse effects from released FA.