Background: A number of reports have been published on both moderately hypofractionated intensity-modulated radiation therapy (IMRT) for localized prostate cancers using 2.4 – 3.4 Gy per fraction over 4 – 6 weeks, and ultrahypofractionation over 1 – 2 weeks using 6 Gy or more per fraction. However, long-term outcomes have sparsely been reported using doses between these two fractionations (3.4 – 6.0 Gy per fraction) over 2 – 3 weeks (high hypofractionation). Therefore, we performed a pilot study of highly hypofractionated IMRT in 15 fractions using 3.6 Gy per fraction over 3 weeks for localized prostate cancer. We now report the long-term outcomes of the study. Methods: Twenty-five patients with low- to intermediate-risk prostate cancer were enrolled in this prospective study from April 2014 to September 2015. Fifty-four Gy in 15 fractions (3.6 Gy per fraction) was delivered over three weeks using IMRT with CBCT-based image guidance without intraprostatic fiducial markers. Based on an estimated alpha/beta ratio for prostate cancer of 1.5 Gy, the EQD2 was 78.7 Gy for the present dose fractionation. Neoadjuvant hormonal therapy (HT) was administered to intermediate-risk patients for 4 – 8 months, but was not mandatory in patients with low-risk disease. Adjuvant HT was not provided for any patient. Biochemical relapse-free survival (BRFS), clinical relapse-free survival (CRFS), overall survival (OS), and the cumulative incidence rate of late toxicities were analyzed using the Kaplan Meier method. Results: Twenty-four patients were treated with highly hypofractionated IMRT, except one case switched to conventionally fractionated IMRT because we could not meet the dose constraint of the small bowel in treatment planning. Among the 24 patients, 17 % had low-risk, and 83% had intermediate-risk disease. Neoadjuvant HT was administered to 23 patients, and the median duration of HT was 5.3 months. The median follow-up period was 77 months (range, 57–87 months). BRFS, CRFS, and OS were 91.7%, 95.8% and 95.8% at 5 years, and 87.5%, 86.3% and 95.8% at 7 years, respectively. No patient died of prostate cancer, but one patient died due to the other cancer. Neither grade ≥2 late gastrointestinal toxicity nor ≥3 late genitourinary toxicity was observed. Grade 2 late genitourinary toxicity did not occur during the first 4 years, but was developed in 5 patients after more than 4-year follow-up. The cumulative incidence rate was 12.9% and 22.8% at 5 and 7 years, respectively. Conclusions: Highly hypofractionated IMRT delivering 54 Gy in 15 fractions over 3 weeks for prostate cancer without intraprostatic fiducial markers demonstrates favorable oncological outcomes with no severe complications. This treatment approach could be a possible alternative of moderate hypofractionation, although further studies will be mandatory.