In a prospective, double-blind, placebo-controlled study, we examined the effect of mountain cedar (MC) immunotherapy on the MC-induced late cutaneous response (LCR). Fourteen MC-sensitive patients were intradermally skin tested before and after immunotherapy with MC extract. We measured the size of the wheal at 15 minutes and the area of tissue swelling at 6 hours. Patients were matched by the size of the LCR and started receiving either MC immunotherapy or placebo immunotherapy. MC-specific immunoglobulins (MC sIgG, MC sIgG1, MC sIgG4, and MC sIgE) were measured by ELISA. Symptom-medication scores (SMSs) were recorded on a daily basis during the MC season and tabulated at the end of the study. Comparison of the 14 paired patients revealed no significant differences beween MC-treated and placebo-treated groups in preimmunotherapy MC sIgG1 and SIgG4. However, when MC immunotherapy was compared to placebo immunotherapy, patients receiving MC immunotherapy developed significantly higher MC sIgG1 (p < 0.04) and MC sIgG4 (p < 0.01) after immunotherapy. Patients receiving MC immunotherapy also demonstrated significantly greater suppression of the LCR after immunotherapy (p < 0.005) with the postimmunotherapy LCR correlating significantly with both MC sIgG4 (rs = 0.715; p = 0.008) and cumulative dose of MC received (rs = 0.808; p = 0.004). MC sIgE was similar in both groups after immunotherapy. The reduction in SMSs in the MC-treated group did not reach significance, nor was there a correlation of SMSs with MC sIgE, sIgG, sIgG1, or sIgG4. In conclusion, this prospective study demonstrated that the LCR in MC-sensitive patients was suppressed by MC immunotherapy and that this suppression was strongly correlated with both MC sIgG4 and cumulative dose of MC administered.