The severity of intestinal disease in the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) (-/−) mice has been reported to co-segregate with gene loci which contain the genes for hydroxysteroid sulphotransferase (SULT). Because of the potential involvement of steroid hormones in CF, we investigated levels of steroid SULT activity in the livers of CFTR mice to determine whether the levels of SULT activity correlate with the occurrence or severity of CF. To elucidate the possible role of SULT activity in ameliorating the deleterious effects of CF in CFTR (-/−) mice, we determined the levels of phenol SULT (PST), hydroxysteroid SULT [dehydroepiandrosterone (DHEA)-ST] and oestrogen SULT (EST) activity in control CFTR (+/+), heterozygous CFTR (+/−) and homozygous CFTR (-/−) mice, which survive to adulthood. The level of PST activity was not significantly different between any of the groups of mice, regardless of sex or genotype. Although DHEA-ST activity was significantly higher in female mice than in male mice, there was no difference in DHEA-ST activity that could be correlated with genotype. In contrast with PST and DHEA-ST activities, we found that some male and all female adult CFTR (-/−) mice had elevated, dramatically different levels of EST from both CFTR (+/+) and CFTR (+/−) mice. Results from these SULT activity experiments were confirmed by Northern-blot analysis of mouse-liver RNA. Subsequent studies with preweanling mice revealed no differences in the levels of EST that could be correlated with genotype. Thus this study indicates that EST is elevated significantly in CFTR (-/−) mice which survive to adulthood and provides important biochemical information that EST levels may be protective in CF.