Benzalkonium chloride (BAC) is a preservative, which is a component of more than 70% of topical ophthalmic drugs. Although BAC is a preservative with an excellent antibacterial spectrum, it has also been shown to induce toxic effects to the ocular surface. Several studies have indicated that BAC may also have altering effects on the bacterial flora of the conjunctiva.

Since dry eye syndrome (DES) is a very common and multifactorial disease of the ocular surface and the tear fluid resulting in tear film instability, inflammation of the ocular surface, symptoms of discomfort and visual impairment, there are many different preserved as well as preservative-free ophthalmic preparations of ocular lubricants for the treatment of DES. Unpublished data from our department shows differences between artificial tears for the treatment of DES with and without BAC in bacterial culture.

Therefore, the aim of this study is to investigate the effect of artificial tear eyedrops with and without BAC on the conjunctival bacterial flora. For this 40 patients with mild or moderate dry eye syndrome with no use of artificial tears in the 4 weeks preceding the study will be recruited and treated either with the preservative-free "Prosicca sine" eyedrops or the BAC containing "Prosicca" eyedrops for one month. Conjunctival samples will be collected of one eye of each patient before and after the 1-month treatment period to compare the conjunctival bacterial flora of the two treatment groups.

Inclusion Criteria:

• Men and women aged between 18 and 75 years.
• Signed and dated written informed consent.
• History of dry eye syndrome for at least 3 months
• Tear Break Up Time (BUT) ≤ 10 seconds or Schirmer I test ≤ 10 mm and ≥ 2mm
• Normal ophthalmic findings except dry eye syndrome, ametropia < 6 Dpt.
• Recommended use of topical lubricants, but no administration of topical lubricants 4 weeks preceding the first study day.

Exclusion Criteria:

• History or presence of ocular disease judged by the investigator as incompatible with the study.
• Any other topical ocular treatment than the study medication in the 4 weeks preceding the first study day and during the treatment period.
• Wearing of contact lenses.
• Symptoms of a clinically relevant illness in the 3 weeks before the first study day.
• Participation in a clinical trial in the 3 weeks preceding the first study day.
• Pregnancy, lactation.