We prospectively followed a cohort of 46 newly diagnosed cases of lupus nephritis (LN) over a mean period of five years in order to determine the renal relapse rate, to identify potential risk factors for relapses, to assess the value of serological tests during flares and to analyse their impact on global outcome. Of the patients 37% experienced at least one renal flare, the first episode occurring after a mean follow-up of 40 months, when most patients were still treated with low-dose glucocorticoids and azathioprine. Baseline biochemical and pathological data did not differ between relapsing and nonrelapsing patients. Serological findings (low complement and high anti-DNA antibody) were less pronounced at relapse. Moreover, neither a decline in complement nor a rise in anti-DNA antibody titres were observed by the time of renal relapse, compared to immediate pre-flare values. Poor outcomes were observed only in relapsing patients. Taken together, renal relapses in LN patients are common, have a negative impact on outcome, but cannot be readily predicted. These results stress the importance of regular blood and urine examinations in LN patients, even years after the initial episode.