Background: As part of the planning for a future multicenter study, this preliminary clinical trial used serum samples from patients to identify biomarkers for predicting the therapeutic effects of sublingual immunotherapy (SLIT) for Japanese cedar pollinosis (JCP). Methods: This prospective study included patients undergoing SLIT for JCP at our hospital. All enrolled patients (N = 17) started SLIT between June and November of 2015. With informed consent from the patients, blood samples were obtained in January, March, and June of 2016, and patients completed the Japan rhino-conjunctivitis quality of life questionnaire (JRQLQ). On the basis of the JRQLQ results, the 6 patients with the best outcomes were included in the high-response group (HRG), and the 5 patients with the worst outcomes were included in the poor-response group (PRG). We then compared serum data between the two groups, to identify useful biomarkers. Results: IL-12p70 and VEGF levels tended to be higher in the HRG than in the PRG in January, March, and June (0.10 > p > 0.05). In addition, the June IL-17 level was significantly higher (p < 0.05) in the HRG than in the PRG. Conclusions: IL-12p70 and VEGF may be useful biomarkers for predicting the effects of SLIT. In addition, although IL-17 does not appear to be useful as a biomarker for evaluating treatment response at the start of SLIT, it may be useful as a biomarker after the beginning phase of SLIT.