Mastocytosis is diagnosed without difficulty if it presents with easily recognizable lesions of urticaria pigmentosa. Recently, we have identified hardly visible skin lesions of mastocytosis in Hymenoptera venom allergic patients ("occult mastocytosis"). In addition, in approximately 15% of the patients with typical cutaneous lesions, urticaria pigmentosa was at first mistaken for other conditions and thus not linked to simultaneous symptoms of systemic mastocytosis. In most patients with unrecognized mastocytosis, the diagnosis was supported by raised basal serum tryptase levels. Cutaneous mastocytosis is often overlooked and more frequent than assumed. Measurement of basal serum tryptase concentrations can make an important contribution to the diagnosis of mastocytosis, but it does not replace a meticulous skin examination.