Background. Testosterone deficiency represents a significant health risk factor for men but the importance has so far been underestimated. Besides physiological and age-related reduction, acquired testosterone deficiency may also occur. Testosterone deficiency is a possible result of commonly occurring diseases or is itself the basis for development of different diseases. The scope of the present investigation was measurement of serum testosterone levels in different age groups. Materials and methods. Serum testosterone levels were determined in samples from 5,735 healthy men at the LADR laboratory MVZ Dr. Kramer & colleagues, Geesthacht under routine conditions. The frequency of testosterone deficiency was calculated in different age groups and compared using SPSS 19.0 software. Results. Pathologically low testosterone levels (<2.5 ng/ml) were found in 15.2% of subjects while 37.4% had a testosterone level lower than 3.5 ng/ml. Decreased testosterone levels were not associated with age. In addition the proportion of men with decreased serum testosterone levels was comparable in all age groups. The average serum testosterone level decreased slightly in all age groups during the period before midday. Conclusions. The data reveal high rates of testosterone deficiency in men independent of patient age. As decreased serum testosterone levels may be the consequence of several diseases and can be causally involved in the pathogenesis of further diseases, it is strongly recommended that serum testosterone measurement should be included in the diagnostic arsenal especially when symptoms, such as loss of libido, erectile dysfunction, lack of concentration, depression, lethargy, irritability and sleep disturbance are present.