Although patch testing is regarded to be the most standardized and reliable method for the diagnosis of delayed-type hypersensitivity to contact allergens, its substitution by in vitro techniques would be advantageous for several reasons. The lymphocyte transformation test (LTT) measures the allergen-specific proliferation or activation of T cells in vitro and has been explored as potential alternative for patch testing for decades. Besides numerous attempts to increase its sensitivity and specificity, the major drawback of a cell culture-based method is the poor solubility of many contact sensitizers and the need for chemical modification of prohaptens. LTT is used with regularity only for the diagnosis of hypersensitivity to beryllium and adverse reactions to drugs. Its usefulness in occupational dermatology is limited and restricted to special situations as a supplement for patch testing with large panels of well-established sensitizers.