We studied here the clinical course of heterozygous carriers of X-linked Alport syndrome and a subgroup of patients with thin basement membrane disease due to heterozygous autosomal recessive Alport mutations whose prognosis may be worse than formerly thought. We analyzed 234 Alport carriers, including 29 with autosomal recessive mutations. Using Kaplan-Meier estimates and log-rank tests, autosomal and X-linked carriers were found to have similar incidences of renal replacement therapy, proteinuria, and impaired creatinine clearance. Further, age at onset of renal replacement therapy did not differ between X-chromosomal and autosomal carriers. Both groups showed an impaired life expectancy when reaching renal replacement therapy. RAAS inhibition significantly delayed the onset of end-stage renal failure. Not only carriers of X-linked Alport mutations but also heterozygous carriers of autosomal recessive mutations were found to have an increased risk for worse renal function. The risk of end-stage renal disease in both groups affected life expectancy, and this should cause a greater alertness toward patients presenting with what has been wrongly termed 'familial benign hematuria.' Timely therapy can help to delay onset of end-stage renal failure. Thus, yearly follow-up by a nephrologist is advised for X-linked Alport carriers and patients with thin basement membrane nephropathy, microalbuminuria, proteinuria, or hypertension.