This study was designed to determine the incidence of relapse and factors predictive for relapse in 719 patients with severe aplastic anaemia (SAA) after immunosuppressive treatment (IS). Patients developing myelodysplasia or acute leukaemia after IS, and patients receiving a transplant, were excluded from this analysis. Response was defined as reaching complete independence from transfusions, relapse was defined as becoming again transfusion dependent. This criteria was validated by similar figures when using other 'relapse criteria' such as drop in neutrophil or platelet counts. Of 358 patients responding to IS. 74 patients relapsed after a mean time of 778 d after treatment. The actuarial incidence of relapse is 35.2% at 14 years after IS. The risk for relapse was higher in patients responding within 120 d from IS (48%) compared to patients responding between 120 and 360 d (40%) and only 20% for slow responders (> 360 d from IS) (P < 0.00001). In multivariate analysis this factor still proved significant (P < 0.0001). The mean time between diagnosis and treatment was significantly longer in patients relapsing compared to patients who did not relapse (260 v 134 d, P = 0.037). Relapse was not predicted by the severity of the disease, age, and sex. In 39 of the 74 relapsing patients a second response could be achieved. Responses after relapse were associated in univariate analysis with early response to previous IS and early occurrence of relapse. The actuarial survival of patients not relapsing is significantly better than survival of patients relapsing (79.8% v 67.1%, P = 0.0024). However, the actuarial survival of 39 relapsing patients who responded again to IS was similar to patients not relapsing (86%) and significantly better than in 35 patients not reaching a second response after relapse (49.3%, P = 0.0015). This study indicates that relapse is a relevant problem in the treatment of aplastic anaemia, and does have an impact on overall survival. Prospective studies of immunosuppressive regimens, looking at responses, should also address this problem in the future.