Background: The accurate measurement of proteinuria is critical to the clinical management of lupus nephritis. The 24-hr urine collection has been commonly used to objectively measure the amount of proteinuria but is a laborious procedure for patients and is prone to cause collection error. Alternatively, spot urine ratio of protein to creatinine (P/Cr) has been introduced as an alternative representation of the 24-h urine collection in various kinds of kidney diseases with proteinuria. However, it is not known whether spot urine P/Cr can replace 24hr urine protein in lupus nephritis. Methods: This is a retrospective study investigating the association of random spot urine P/Cr and 24hr urine protein in patients with biopsy-proven lupus nephritis. A total of 286 samples of random spot urine and concurrent 24hr-urine collections from 120 patients were analyzed. All the patients were diagnosed as systemic lupus erythematosus according to the revised criteria of American College of Rheumatology and were enrolled from Seoul National University Hospital between May 2004 and July 2010. Results: In 286 selected samples, 24 hr urine protein excretion ranged from 42 mg/day to 36,080 mg/day. The range of 24 hr urine P/Cr was 0.04 to 49.72. Random urine P/Cr ranged from 0.06 to 51.40. There was a strong correlation between 24 hr urine protein excretion and random urine P/Cr (r=0.774, p<0.001). A strong correlation (r=0.939, p<0.001) was also found between 24hr urine P/Cr and random P/Cr. The spot urine P/Cr was correlated well with 24hr urineprotein until 24 hr urine protein is up to 8 g/day (r=0.742, p<0.001). This result shows that random spot urine P/Cr is a reliable substitute of 24 h urine protein excretion in a wide range of urine protein excretion. Conclusion: This study shows that random spot urine P/Cr is well correlated with 24 hr urine protein in a wide range of 24hr urine protein excretion. The result supports the use of random spot urine P/Cr inb screening and monitoring of proteinuria in patients with lupus nephritis.