OBJECTIVE To determine whether patients with idiopathic systemic lupus erythematosus (SLE) are associated with impaired CYP2D6 activity and to gain insight into whether there is an association between particular CYP2D6 genotypes and susceptibility to SLE, and whether CYP2D6 polymorphism is linked to any specific clinical features of SLE. METHODS Debrisoquine sulfate (10 mg p.o.) was given to 159 healthy volunteers and 39 idiopathic SLE patients. Genotypic assay was carried out in 80 healthy volunteers and 32 patients. A 10-ml blood sample was drawn for genotypic assay. Debrisoquine and 4-hydroxydebrisoquine were determined in 8-h urine samples. Blood samples were analysed for the presence of mutations in the CYP2D6 gene, by using polymerase chain reaction (PCR) specific for CYP2D6*3 and CYP2D6*4 alleles. RESULTS The metabolic ratio of debrisoquine to 4-hydroxydebrisoquine ranged from 0.01 to 86.98 in healthy subjects and from 0.02 to 96 in SLE patients. We observed the poor metabolizer(PM) debrisoquine phenotype in three of 39 patients with idiopathic SLE (7.6%) and five of 159 healthy subjects (3.1%). There was no significant difference in the frequency of PM phenotypes between idiopathic SLE and healthy subjects (Fisher's exact test, P = 0.19). No significant difference in the distribution of overall genotypes and allele frequencies were observed between the two groups. No significant relationships were found between specific clinical features and the overall genotype. CONCLUSIONS The results of this study confirm that CYP2D6 activity is not impaired in SLE and that there is no association between SLE and phenotypic CYP2D6 status. The results also showed that there was no difference in the frequency of CYP2D6A and CYP2D6B alleles between controls and patients with SLE.