Acute and chronic side-effects have been reported during topical treatment of mycosis fungoides with nitrogen mustard (HN2). In order to estimate the risk for the nurse applying topical HN2, the concentration of HN2 in the air during treatment was measured. Air samples were obtained before, and during the 20-min treatment period close to the nurse and patient, and continuously at a distance of 1 m from the patient. The mean concentration of HN2 in the air during treatment was 0.036 mg/m3. Immediately after treatment the concentration dropped to 0.004 mg/m3. The mean cumulative concentration in the room during the 3 1/2 h of the experiment was 0.012 mg/m3. If the MAC (maximal allowable concentration) value of 0.05 mg/m3 for the comparable sulphur mustard is used as a guideline, then with a treatment procedure of 20 min this level is not exceeded. Nevertheless, it is important to minimize the exposure of nursing staff to HN2.