Occupational exposure of floriculturists is characterized by alternating periods of intense pesticide spraying and reduced or no activity. Induction of sister chromatid exchanges (SCE), structural chromosome aberrations (CA) and micronuclei (MN) was investigated in peripheral lymphocytes of a group of 23 Italian floriculturists and 22 matched controls. Blood sampling was performed during and one month after the end of intensive pesticide treatments, in order to cover a period of high and low exposure, respectively. Each donor was genotyped for glutathione S-transferase M1 (GSTM1), T1 (GSTT1), and N-acetyltransferase 2 (NAT2), three polymorphic genes involved in xenobiotic metabolism, to assess their potential role in individual genotoxic response to the pesticide exposure. No effect of the pesticide exposure on the cytogenetic parameters were detected. Smoking, however, was found to increase SCE levels. The only significant influence of phenotype composition on cytogenetic response was an increase in SCE levels in the GSTT1 positive individuals compared with the GSTT1 nulls (P=0.02). This finding was, however, based on only four GSTT1 null donors (n=41 for GSTT1 positive donors). In addition, a possible interaction was observed between smoking and GSTM1 genotype in the CA assay, GSTM1 null smokers, earlier reported to have an elevated risk for lung cancer, showing higher CA frequencies than GSTM1 positive smokers.