Background: keratoconus is bilateral (usually asymmetrical), progressive, non inflammatory, cone like anterior protrusion of the cornea involving the central and the inferior para central areas that results in corneal ectasia, astigmatism and diminution of vision. It usually seen after puberty, with incidence of 1 in 2000 of general population, the major benefit of corneal tomography is the measurement of the posterior corneal surface, because the posterior surface contributes minimally to the overall refractive power of the eye (due to the minimal difference between the index of refraction of the cornea and aqueous) it was considered less important both diagnostically and therapeutically. The posterior cornea, however, is an earlier indicator of ectatic change or ectasia susceptibility. Aim of the Work: to determine the changes of posterior corneal astigmatism in cases with keratoconus. Patients and Methods: one hundred eyes of 50 patients were included in this retrospective, case control study which adhered to the tenets of the Declaration of Helsinki and was approved by the Ethics Committees of the Ain Shams University Hospitals. All patients included in the study provided informed consent, these cases were divided into: 1- Keratoconus group: comprised 50 eyes of 25 patients with keratoconus diagnosed on the basis of clinical and topographic signs and 2- Control group comprised 50 eyes of 25 normal candidates for refractive surgery. Results: the mean magnitudes posterior corneal astigmatism(PCA) were approximately 1 D, and The PCA values were significantly associated with the severity ofKeratoconus(KC), the posterior corneal surface was more affected in the early stages of KC. Conclusion: we found that the mean magnitudes PCA were approximately 1 D, and The PCA values were significantly associated with the severity of KC, the posterior corneal surface was more affected in the early stages of KC.