Alcohol and tobacco addictions are often joined. There are biological and environmental reasons of the co-addiction. Active compounds present in the substances can interact in different ways, for example in biotransformation reactions. The aim of the paper was to describe differences of ethanol and nicotine metabolites concentrations in biological fluids. The material was blood and urine collected from alcohol and tobacco addicted patients treated in The Detoxication Department, Toxicology Clinic in Krakow. Acetaldehyde and volatile substances were determined in blood by GC-FID. Cotinine was determined in urine by HPLC-UV. Most (91.3%) of alcohol addicted patients were tobacco smokers. In the group 76.3% patients smoked more than 11 cigarettes per day. Tobacco addiction was usually longer or lasted the same time as alcohol addiction. Mean cotinine urine concentrations in the group of alcohol addicted smoking patients were lower than in group of smoking patients not alcohol addicted (p < 0.05). Differences between mean acetaldehyde blood concentrations in group of alcohol addicted smoking patients and alcohol addicted non smoking patients were not significant (p > 0.05).