AIM: To compare the tear film quantity and stability parameters in keratoconus (KCN) and normal eyes using test breakup time (TBUT), noninvasive TBUT (NITBUT), and Schirmer test. METHODS: All participants (n=166), including patients with KCN and age-matched healthy individuals with normal corneas, were recruited from those referred to Farabi Eye Hospital, Iran, in 2023. To better account for genetic and environmental factors, the control group comprised healthy individuals who were relatives of KCN patients and had normal corneal topography. Tear quantity parameters were evaluated in the following order: NITBUT, TBUT, and Schirmer tests. RESULTS: The mean age of cases in KCN (61.7% males) and normal (63.5% males) participants was 27.54±5.44y (range 19 to 38) and 27.52±5.63y (range 20 to 38), respectively (P=0.976). NIBUT, TBUT, and Schirmer’s tests were significantly lower in KCN group compared to normal controls (all P<0.001). The mean difference for NIBUT was -7.81s (P<0.001), and for TBUT was -7.61s (P<0.001). Schirmer test values were also significantly lower in the KCN group, with a mean difference of -5.61 mm compared to normal people (P<0.001). CONCLUSION: Our findings demonstrate significant tear film impairment in KCN. The reductions in NIBUT, TBUT and Schirmer scores highlight an underlying tear film dysfunction in KCN that extends beyond the morphological changes of the cornea.