OBJECTIVE To investigate whether formocresol, in Buckley's original formulation, is mutagenic in vivo to lymphocyte cultures obtained from the peripheral blood of children aged from 5 to 10 years old. These children were recruited from those attending the dental clinics of Recife City Council and the University of Pernambuco School of Dentistry, Brazil. METHODS The sample comprised 20 children who had primary teeth with cariously exposed vital pulps. Two venous blood samples were collected (6-8 ml) from each child, the first prior to vital pulpotomy (control group) and the second 24 h after pulpotomy (treated group). This research is a case-control study. The peripheral lymphocytes were grown in a complete culture medium consisting of 78% RPMI 1640 medium (a), supplemented with streptomycin (0.01 mg/ml), penicillin (0.005 ml(-1)), 20% fetal bovine serum (b) and 2% phytohemagglutinin (c). The lymphocytes were assessed for chromosomal aberrations via a previously published method which was modified. The cytogenetic analysis was performed in a blind test, where the slides were codified by an annotator and the scorers did not know which group they were analyzing. For each sample, this envolved the analysis of 200 metaphases. The level of significance adopted in the statistical test was 5.0% (p<0.05). RESULTS There was no statistically significant difference in clinical doses between the control and treated groups, using Wilcoxon's Signed Ranks test, for the chromosomal aberrations (P=0.251) and for the total chromosomal breaks (P=0.149). Although there were no statistically significant differences between the control and treated groups, Buckley's formocresol was mutagenic for one patient, raising doubt about the desirability of its use for pulpotomies in children. CONCLUSIONS The results revealed that, from a statistical standpoint, formocresol is not mutagenic. However, further investigations are required, preferably with a larger sample, in patients needing more than one pulpotomy in order to observe whether an increase in the quantity of the drug would increase the quantity of chromosome aberrations and also to verify individual susceptibility to chromosome alterations with the use of formocresol.