Polychlorinated biphenyl (PCB) and ΣDDT (i.e. p,p′-DDT + p,p′-DDE + p,p′DDD) concentrations were determined from the analysis of 115 Welsh breast milk samples collected in 1990 and 1991. Fifty PCB congeners were screened, of which 24 were identified in most samples. The PCB congener pattern was consistent between individual milk samples, with IUPAC congeners 28, 138, 153 and 180 being the most abundant and accounting for an average of 50% of the ΣPCB concentrations determined. PCB concentrations varied between 2 and 70 ng g−1 whole milk, were positively correlated with age, and negatively correlated with the total lactation period and with the percent lipid content of the milk. PCB pattern distributions differed between milk and adipose tissue samples. Human milk had a higher proportion of tri- (18 and 28), tetra- (44, 52 and 66) and pentachlorinated biphenyls (101) compared to human adipose tissue. ΣDDT concentrations ranged from 0·3 to 71 ng g−1 of whole milk, with p,p′-DDE contributing towards an average of 92% of the ΣDDT concentrations. ΣDDT levels were also positively correlated with age and negatively associated with the lactation period, though these correlations were rather weak. No significant differences in the ΣPCB and ΣDDT concentrations were noted between milk samples from donors living in rural and urban locations, or between the subjects' body weight, smoking habits or diet.