The alkaline comet assay was employed in the assessment of DNA damage in leukocytes of a worker incidently exposed to gamma radiation (221 mSv, 60Co source). The comet tail lengths and tail moments were studied. By using the alkaline comet assay immediately after accidental exposure a high level of DNA damage was recorded. The highest levels of DNA damage were recorded one day and one week after the radiation incident. Later on, a decrease in both comet parameters was observed. Although the level of DNA damage was diminished during a one year period, it was still elevated compared to normal values recorded in leukocytes of a healthy, unexposed person. The results obtained indicate that the alkaline comet assay is a rapid and sensitive microdosimetric technique and is suitable for in vivo human biomonitoring, especially in cases of incidental exposure to ionising radiation.