We studied the DHAS concentrations in breast duct fluids obtained by nipple aspiration from 73 healthy non-lactating women and 23 women with gross cystic disease (GCD) of the breast. The presence of DHAS in breast fluid is age-related: no DHAS could be detected in breast fluid in 50% of healthy subjects aged 20-30 years and in 29% of healthy subjects aged 31-40 years. All healthy subjects aged 41-50 years showed DHAS in breast fluid. All but two fluids obtained from GCD patients (age range 20-40 years) contain DHAS. Moreover the DHAS concentration in this group was higher than in age-matched controls (P less than 0.01). Higher DHAS levels in GCD patients may thus be regarded as a factor modifying the mammary hormonal environment, possibly involved in the clinical course of the disease.