The rate of deterioration of renal function was determined in 29 patients with juvenile nephronophthisis presenting before the age of 18.5 years. The analysis was based on 618 serial determinations of serum creatinine (SCR). By application of a new statistical method the time elapsing between different successive SCR levels was calculated. A more or less homogeneous increase of SCR was noted starting from the time when a SCR of 4 mg/dl was reached. The median time elapsing between an SCR of 2 and 4 mg/dl was 32 months, between 4 and 6 mg/dl 10 months and between 6 and 8 mg/dl 5 months. The median age at the start of renal replacement therapy or death due to uraemia, evaluated by survival analysis, was similar in both sexes.