Therapeutic drug monitoring (TDM) for lithium is supported on the basis of clearly defi ned therapeutic range. TDM is of particular importance in individuals whose pharmacokinetic behaviour is changing as the result of aging and maturation. The objective of our retrospective study was to evaluate possible association between the lithium concentration and patient age. The study sample was drawn from a data set of 289 patients who had serum lithium estimations between March 2004 and April 2006. 88% of 532 results were maintained at therapeutic range (0.3-1.3 mmol/l) and 10% results were lower than 0.3 mmol/l. There was no difference in the age-dependent distribution of results between females and males. The patients aged 51-60 years had the greatest percentages of results lower than 0.3 mmol/l. Most of the patients (59%) had only single serum lithium estimation. The increased numbers of serum lithium estimations was associated with greater percentages of lithium concentration at therapeutic range (91 vs. 83). The results of our analysis show the signifi - cant part of patients receiving lithium has serum lithium concentration below the therapeutic range. Regular monitoring of lithium serum concentration is likely to improve treatment adherence.