Two similar cases of rapidly progressive fibrosing interstitial nephritis in young women who followed the same slimming regimen prompted us to conduct an epidemiological survey of the nephrology centres of Brussels and to further investigate the exact nature of this slimming treatment. Seven other women under the age of 50 in terminal or preterminal renal failure were admitted for dialysis in 1991 and 1992. They had all followed a slimming regimen in the same medical clinic. Renal biopsy samples in eight of the nine cases showed extensive interstitial fibrosis without glomerular lesions. Two of the patients were seen for the first time in terminal renal failure and were started immediately on dialysis. For the seven other women, the nephropathy was characterised by a rapid deterioration in renal function, with initial serum creatinine doubling within about 3 months. The clinic had specialised in slimming treatments for the previous 15 years without any problems. In May, 1990, therapy was changed, with the introduction of two Chinese herbs (Stephania tetrandra and Magnolia officinalis). In June, 1992, three of twenty-five randomly selected women who had followed the same regimen during at least 3 months from 1990 had impaired renal function. Chemical analysis of some brands of these Chinese herbs did not show nephrotoxic contaminants of fungal or plant origin (ochratoxin or aristolochic acid) or adulteration by diuretics or antiinflammatory drugs. However, the medicinal preparation of the capsules taken by patients had different alkaloid profiles from those expected in Chinese plants. The striking relation between a specific type of fibrosing interstitial nephritis in young women and a slimming treatment involving Chinese herbs adds support to the arguments against uncontrolled therapy with herbal preparations.