Near-infrared spectroscopy (NIRS) is an optical imaging method, which allows non-invasive in vivo measurements of changes in the concentration of oxygenated (O2Hb) and deoxygenated (HHb) haemoglobin in cortical tissue. For the present study, we examined 13 adult ADHD patients and 13 age- and gender-matched healthy controls by means of multi-channel NIRS (Optical Topography; ETG-100, Hitachi Medical Co., Japan) during performance of a working memory (n-back) paradigm. Compared to the healthy control group, ADHD patients showed reduced task-related increases in the concentration of O2Hb in NIRS channels located over the ventro-lateral prefrontal cortex, indicating reduced activation during performance of the n-back task in this part of the brain. This finding was particularly apparent for the task condition with high working memory load (2-back), and was accompanied by a statistical trend towards an increased number of omission errors in the patient group. The data confirm previous findings of working memory deficits and prefrontal cortex dysfunction in patients suffering from ADHD, and are discussed in the light of imaging findings and theoretical models of working memory function.