Increased basal serum tryptase (bsT) levels are a well-described risk factor for Hymenoptera venom-induced anaphylaxis (HVAn) in patients allergic to Hymenoptera venom. Increased bsT levels might also indicate the presence of mastocytosis. In this study we evaluated whether the risk of HVAn increases with increasing mast cell load in patients with mastocytosis. Consecutive patients with different subtypes of mastocytosis (n = 329) admitted to the University Medical Center Groningen were retrospectively assessed. As markers for mast cell load, levels of both bsT and the urinary histamine metabolites methylhistamine and methylimidazole acetic acid (MIMA) were used. In the entire patient group, irrespective of disease subtype and Hymenoptera venom exposure, HVAn prevalence gradually increased with increasing marker levels to a maximum of 36% to 47% at a bsT level of 28.0 μg/L, a methylhistamine level of 231.0 μmol/mol creatinine, and a MIMA level of 2.7 mmol/mol creatinine but decreased thereafter with a further increase in these levels. In patients with indolent systemic mastocytosis with a history of Hymenoptera venom exposure after age 15 years or greater (n = 152), MIMA and age at the most recent Hymenoptera sting were independent predictors for HVAn (odds ratios of 0.723 [P = .001] and 1.062 [P < .001], respectively). In patients with mastocytosis, HVAn prevalence does not increase constantly with increasing levels of mast cell load parameters: after a gradual increase to a maximum of near 50%, it decreases with a further increase in these levels. In the indolent systemic mastocytosis population, all mast cell load markers were independent negative predictors of HVAn. These findings suggest a complex pathophysiologic association between mast cell load and HVAn risk in patients with mastocytosis.