Background: Renal diet knowledge -especially phosphorus knowledge- is generally poor in Maintenance Hemodialysis (MHD) patients due to the complexity of information about dietary phosphorus thus causing poor compliance to dietary phosphorus guidelines and hyperphosphatemia. Objective(s): This study aimed to investigate the relationship between knowledge about phosphorus, dietary phosphorus intake and serum phosphorus level in MHD patients. Methods: A cross-sectional study was conducted on 110 MHD patients in the hemodialysis center in Alexandria Main University Hospital. A questionnaire was used to collect demographic and clinical data and 24-hour dietary recall. A 25-item nutrition knowledge questionnaire was asked to patients. Patients' medical records were reviewed to collect data about the underlying cause of End Stage Renal Disease (ESRD), duration of dialysis and serum biochemical parameters. Dry Body Weight (DBW) and height were measured using a stadiometer following a standard protocol and Body Mass Index (BMI) was calculated. Results: Hyperphosphatemia was common (44.5%) in the present sample. Energy intake and protein intake were inadequate in 76.4% and 74.5% of patients. Mean phosphorus intake of hyperphosphatemia patients was higher than controlled patients (1053.99±420.18 mg/d vs 854.18±353.99 mg/d). Almost half of the patients (51.2 %) who consumed phosphorus intake >1000 mg/d had hyperphosphatemia and 61.0 % of patients with poor total renal diet knowledge and two thirds of patients (66.7%) with poor phosphorus knowledge had hyperphosphatemia. Phosphorus knowledge was a strong determinant for controlled serum phosphorus (OR =0.545 p=0.005), dietary phosphorus intake was a risk factor for hyperphosphatemia (OR=1.001, p=0.024). The main determinant of dietary phosphorus adherence was protein intake (OR =1.084, p < 0.001). Conclusion: MHD patients have both poor renal diet and phosphorus knowledge. Phosphorus knowledge protects against development of hyperphosphatemia. Patients with higher protein intake had a high phosphorus intake. MHD patients need to be educated how to consume adequate protein while choosing lower phosphorus protein choices.