There are few published data on the recovery of fertility after 'little' Bu-Cy (busulfan 16 mg/kg, cyclophosphamide 120 mg/kg) conditioning for BMT. To address this, we identified 19 females aged less than 40 years at transplant and 47 males from a single centre who were alive a minimum of 2 years after BMT with little Bu-Cy as conditioning and who were evaluable for testing. FSH, LH, testosterone and inhibin B levels were measured in males. Twenty-six also had semen analysis, a median of 5 years post transplant; 21 had detectable sperm, with 11 having counts >20 x 10(6)/ml. There was an association between prolonged chronic graft-versus-host disease and low sperm counts. FSH and inhibin B levels correlated with sperm counts but not to the extent that they could reliably predict counts in individual patients. An additional six of seven males attempting to father children did so, a median of 3.2 years post transplant. Low testosterone levels were noted in 12% of males, most of whom had symptoms consistent with androgen deficiency. FSH, LH and oestradiol levels in the absence of hormone replacement therapy were measured in females; all remained amenorrheic with endocrine evidence of ovarian failure. These results have implications for fertility counselling and hormone replacement therapy both pre- and post BMT.