This paper studies a cohort of randomly selected attenders at a district alcohol treatment service and examines the relationship between clinical assessments and laboratory markers currently in use in the unit. It measures in particular the rate of change of serial gamma-glutamyl transferase (GGT) during abstinence throughout an alcohol treatment programme in alcohol-dependent subjects. The results show that GGT is less often elevated in alcohol-dependent patients than was previously thought. Its predictive value changes little with respect to the age of the subject and length of drinking history. Measurement of GGT adds little to the diagnostic sensitivity of careful history taking. In the alcohol-dependent population, GGT estimation is of little value and a normal GGT does not exclude chronic alcohol dependence.