Introduction: The most common hereditary kidney condition is autosomal dominant polycystic kidney disease. It is the cause of 5–10% of end-stage renal disease. Its symptoms are generally late-onset, typically leading to development of hypertension and chronic kidney disease. Ultrasonography is the imaging modality of choice in its diagnosis and management. The aim of this study is to determine the diagnostic value of grayscale ultrasound imaging in evaluating disease severity. Materials and methods: The study group consisted of 81 patients diagnosed with autosomal dominant polycystic kidney disease, 35 adults and 46 children. Inclusion criterion for adults was the presence of at least 10 large cysts in each kidney; children included into the study had developed at least 1 large renal cyst in each kidney. The number of large cysts, echogenicity of kidney parenchyma, cortical thickness and presentation of cortex/medulla boundary were assessed with the use of Logiq E9 apparatus (GE Healthcare, Netherlands). Patients were divided into groups, based on these morphological parameters. Kidney function was assessed according to serum creatinine concentration and creatinine clearance. Statistical analysis was performed, with p-value lower than 0.05 considered as significant. Results: The number of cysts and the degree of parenchymal dysfunction were the determinants of creatinine level and creatinine clearance, with the second predictor proving stronger. Conclusions: We recommend that an ultrasound kidney examination in patients with polycystic kidney disease should include evaluating renal parenchyma and the number of cysts for better assessment of disease severity.