N-Acetylcysteine (NAC) is a free radical scavenger and could therefore act as a radioprotector. To test the feasibility of administering NAC in combination with radiotherapy, we studied 10 patients with inoperable non-small cell lung cancer who were receiving hyperfractionated radiotherapy (RT) of 1.25 Gy B.I.D. (6-h interval) up to a total dose of 60 Gy/48 fractions/32 days. They were given NAC concomitantly with RT: 100 mg/kg i.v. 30 min before the first RT session followed by 30 mg/kg as an i.v. infusion over 7 h; and 600 mg inhaled 30 min before and after each RT session. The patients were assessed by serial CT scans and lung function studies during a 1-year follow-up period. The treatment regime was feasible, but expensive in time and resources. Normal tissue reactions and tumour responses were similar to those in a control group.