Better results following bone marrow transplantation (BMT) have increased the number of patients in longterm follow-up. Health status and late effects are therefore of increasing interest. We discuss here the incidence and follow-up of late complications after allogenic BMT. Any conditioning regimen including TBI in postpubertal women introduces a postmenopausal status. In contrast, normal function is found after BMT without irradiation. In men, there is usually permanent azoospermia after TBI but not following chemotherapy alone. Thyroid dysfunction after BMT with TBI has been reported in about 40% of patients. In most of them compensated hypothyroidism with elevated TSH is found. BMT with TBI induces growth retardation in children. Decreased growth hormone levels are observed and substitution can induce normal height development. Interstitial pneumonitis (IP) is a major cause of death. It occurs on average three months after BMT but late onset IP's are increasingly reported. Cataracts are common after BMT. The risk of cataracts in patients given single dose TBI attains 80-100% after four years. Under treatment with cyclophosphamide alone, the risk of lens opacification is less than 20%. So far, patients treated with fractionated TBI have not developed more cataracts than those treated with chemotherapy alone. Impaired renal function is common early post-transplant. Risk factors include cyclosporine (CSA) nephrotoxic antibiotics and antifungal drugs. So far, few secondary malignancies have been reported in humans after BMT. However, actual observation time is still too short for final evaluation. Prevention or treatment is instituted for common complications. It is expected that other types of tissue damage will be recognized.(ABSTRACT TRUNCATED AT 250 WORDS)