Aflatoxins are potent toxins and carcinogens which can be excreted in the milk of exposed lactating mothers mainly in the form of aflatoxin M(1) (AFM(1)). We previously evaluated the level and frequency of AFM(1) in breast milk in a group of Egyptian mothers attending the New El-Qalyub Hospital, Qalyubiyah governorate, Egypt. In this study, fifty of those women who were AFM(1) positive were revisited monthly for 12 months to assess the temporal variation in breast milk AFM(1). AFM(1) was detected in 248 of 443 (56%) samples. In a multilevel model of the data there was a highly significant (p<0.001) effect of month of sampling on the frequency of AFM(1) detection with summer months having the highest frequency (>80%) and winter months the lowest frequency (<20%) of detection. AFM(1) was observed most frequently in June [OR 63, 95% CI (7.6, 522)]. The level of AFM(1) detection also followed this seasonal pattern with highest mean level in July (64 pg/ml milk, range 6.3-497 pg/ml milk) and the lowest mean level in January (8 pg/ml milk, range 4.2-108 pg/ml milk). The duration of lactation [p=0.0035, OR=1.08, 95% CI (1.02, 1.13)], and peanut consumption [p=0.06, OR=1.69, 95% CI (0.9, 2.9)] also contributed to the model. The identification and understanding of factors determining the presence of toxicants in human milk is important and may provide a knowledge driven basis for controlling the transfer of chemicals to infants.