This study reports the smoking habits in a Danish population, evaluates plasma cotinine and thiocyanate levels in the detection of 'slips' (subjects participating in smoking cessation trials who begin to smoke a few cigarettes per week) and provides distribution scores on questionnaire measures of nicotine dependence. A total of 599 subjects with a mean age of 41 years participated in the study. Of these 46% were current smokers with no difference in the proportion of males(46%) and females(45%) and with a mean cigarette consumption of 12.7 daily. Plasma samples were analyzed for cotinine and thiocyanate, and the smokers completed two questionnaires to measure nicotine dependence: the Fagerström Tolerance Questionnaire and the modified Horn-Russell scale. The mean plasma cotinine was 207 micrograms/l for smokers, 14.4 micrograms/l for occasional smokers and 8.0 micrograms/l for non-smokers (ex-smokers and never-smokers). For plasma thiocyanate the levels were 130 mg/l, 54.8 mg/l, and 54.3 mg/l, respectively. The mean Horn-Russell score was 7.4 and the mean Fagerström score was 5.9. The two tests correlated with a t-value of 0.61 (p < 0.001) and the scores in both tests increased with increasing cigarette consumption. In conclusion, 75% of the smokers consumed 10 or more cigarettes per day and males smoked more cigarettes than females. It was impossible to distinguish occasional smokers (slips) from non-smokers using plasma cotinine or thiocyanate levels. We suggest that studies are needed to evaluate if light smokers benefit from nicotine replacement therapy because they achieve plasma cotinine levels which are similar to those seen when using patches for nicotine replacement therapy.