Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disorder and leads to formation of kidney cysts, kidney enlargement and kidney failure. We present a male patient initially misdiagnosed with ADPKD, partly based on his family history, who was later diagnosed with digenic Alport syndrome (AS) caused by pathogenic variants in COL4A4 and COL4A5. Digenic AS was subsequently diagnosed in three of his sisters, one of whom had previously been diagnosed with ADPKD, due to a de novo heterozygous pathogenic variant in PKD1.