OBJECTIVE The purpose of this study was to assess whether the preservative benzalkonium chloride (BAC 0.01%) present in timolol induced a decrease in basal tear turnover and a deterioration of precorneal tear film in patients with glaucoma and ocular hypertension using topical timolol. METHODS The basal tear turnover of 20 patients with open angle glaucoma or ocular hypertension was measured by computerised objective fluorophotometry when using topical timolol preserved with BAC and 2 weeks after changing to topical timolol containing no preservative. Evaluation of the precorneal tear film was done by measuring the break up time (BUT) before and 2 weeks after changing medication. RESULTS The tear turnover of the patients before the change was 32% lower than that of healthy controls (mean tear turnover values (SD) (%/min): 10.7 (3.0) and 15.6 (5.4), respectively, p < 0.0001). A mean increase of 28% (47%) in the individual tear turnover values was found after the change to the preservative-free timolol (p = 0.04). The BUT values before the change of medication did not differ significantly from those after the change (p = 0.5) but both values were significantly lower than the values of healthy controls (p = 0.009 and p = 0.003, respectively). CONCLUSIONS Preservative-free timolol solution has a favourable effect on the tear turnover of patients with glaucoma and ocular hypertension in comparison with timolol containing BAC. The integrity of the precorneal tear film persisted to be affected when using timolol without BAC. Timolol without preservative can be recommended in those patients who have keratoconjunctivitis sicca or a borderline tear production since BAC may exacerbate a dry eye state.