A variable energy proton accelerator was commissioned at Fermi National Accelerator Laboratory for use in cancer treatment at the Loma Linda University Medical Center. The advantages of precise dose localization by proton therapy, while sparing nearby healthy tissue, are well documented [R. R. Wilson, Radiology 47, 487 (1946); M. Wagner, Med. Phys. 9, 749 (1982); M. Goitein and F. Chen, Med. Phys. 10, 831 (1983)]. One of the components of the proton therapy facility is a beam delivery system capable of delivering precise dose distributions to the target volume in the patient. To this end, a prototype beam delivery system was tested during the accelerator’s commissioning period. The beam delivery system consisted of a beam spreading device to produce a large, uniform field, a range modulator to generate a spread out Bragg peak (SOBP), and various beam detectors to measure intensity, beam centering, and dose distributions. The beam delivery system provided a uniform proton dose distribution in a cylindrical volume of 20‐cm‐diam area and 9‐cm depth. The dose variations throughout the target volume were found to be less then ±5%. Modifications in the range modulator should reduce this considerably. The central axis dose rate in the region of the SOBP was found to be 0.4 cGy/spill with an incident beam intensity of 6.7×109 protons/spill. With an accelerator repetition rate of 30 spills/min and expected intensity of 2.5×1010 protons/spills for patient treatment, this system can provide 50 cGy/min for a 20‐cm‐diam field and 9‐cm range modulation. The distal edge of the spread out Bragg peak was observed at 27.5‐cm depth with an incident proton energy of 235 MeV. The dose at the distal edge falls from 90% to 10% of peak value in 7 mm. The 90%–10% penumbras as small as 6 mm were measured at 3‐cm depth without range shifter material in the beam and as large as 15 mm when measured at 25‐cm depth with 10 cm of range shifter placed in the beam. The unmodulated Bragg curve had a peak to entrance dose ratio of 3, while the peak to entrance dose ratio for the 9‐cm spread out Bragg peak was observed to be only 1.3.