Background: The objective of this trial was to evaluate the efficacy and safety of immunochemotherapy with very low-dose interleukin-2, interferon-alpha and tegafur uracil (IAT therapy) in treatment naïve metastatic renal cell carcinoma (RCC) patients with pulmonary metastasis only. Methods: Previously untreated metastatic renal cell cancer patients only with pulmonary metastasis were enrolled in this multicenter, open-label, single arm, phase II trial. Eligibility criteria included ECOG performance status (PS) 0-2, low or intermediate risk per MSKCC score, and adequate organ function. The primary endpoint was the overall response rate (ORR) per RECIST v1.1. Secondary endpoints were time to progression (TTP), response duration, overall survival (OS), ECOGPS and safety. Safety was assessed by CTCAE, v4.0. Low dose (0.7-2.1 MIU/ day) IL-2 was administered intravenously in the first five days of the first and third week. IFN-alpha (500 MIU/day) was administered 3 times per week in week 2. From 4th to 7th weeks, patients received IFN- alpha 3 times per week and 100 mg tegafur uracil t.i.d. Results: Twenty-eight eligible patients were enrolled from Apr 2009 through Dec 2013. Twenty-one patients were evaluable for ORR. Three patients had partial response (PR) 11 stable disease (SD), and 7 progressive disease (PD). ORR was 14.4% and median duration of PR was 18.0 months. TTP and OS were not reached to median and 3-year OS rate was 76.3%. Twenty-seven patients (96.4%) had an ECOGPS of 0 and 1 (3.6%) had PS of 1. Three patients experienced exacerbation in PS (PS 0 to 1). The most common adverse events (AEs) were elevation of ALT (35.7%) and fever (32.1%). AEs grade 3 included rash (14.3%), depression (3.2%), syncope (3.2%), fever (3.2%), elevation of ALT (3.2%). Four patients discontinued therapy at the time of ORR analysis due to AEs (3 rash, 1 depression). Conclusions: IAT therapy for RCC patients with only pulmonary metastasis is safe and appears to have clinical benefit as reflected by ORR rate and long OS. Clinical trial information: 00002222.