A boy, aged 14, suffered since his 5th year from nephrotic syndrome. During the first 7 years he had been treated with adrenocorticosteroids, but his condition hardly improved. Then a cyclophosphamide treatment was started with a total dosis of 5 g in 50 days. The nephrotic syndrome had disappeared, but relapsed 3 years afterwards, this time accompanied by chronic myelogenous leukaemia. The possibility of the onset of chronic myelogenous leukaemia in connection with the immunosuppressive therapy (adrenocorticosteroids and cyclophosphamide) is discussed.