Immunoglobulin heavy chain gene (IgH gene) rearrangements are found in the majority of patients with B lineage acute lymphoblastic leukaemia (ALL). Two hundred and three bone marrow samples from 54 patients (33 adults and 21 children) were analysed by PCR within specific time-points after diagnosis (ie 1, 2-3, 4-6 and 7-12 months) using FR1 and JH primers (fingerprinting with a sensitivity > or =1:5 x 10[3]). CDR3-derived allele specific oligoprimers (ASO to achieve a sensitivity between 1:10[4] and 1:10[5]) were applied to 12 children and 18 adults, while size of CDR3 region, oligoclonality and background problems prevented their application to the remaining patients. All patients were followed clinically for > or =24 months. Thirty adults and 16 children presented as newly diagnosed ALL, while the remaining eight patients were analysed in first or subsequent relapse. Patients destined to relapse showed a higher proportion of positive tests (> or =50%), particularly after 1 month, than in the remission group, irrespective of age. Among patients staying in remission, a decrease in MRD-positive tests occurred during the first 12 months in both age groups. However, the proportion of positive tests dropped below 15% at a later stage in adults (4-6 months) than in children (2-3 months). Among children, only patients destined to relapse were MRD positive beyond 1 month, with the exception of only one patient, still positive at 2-3 months in the remission group. The difference in MRD positivity between relapse and remission patients was statistically significant in children (P < 0.03) at any time of testing, but only at 4-6 months in adults (P < 0.01). These data suggest that resolution of MRD in ALL occurs more rapidly in children compared to adults, particularly within the first 6 months. Children and adults, studied in first or subsequent relapse, showed a higher proportion of positive tests during reinduction compared to newly diagnosed patients.