232 human primary invasive breast tumors were analyzed with 13 polymorphic microsatellite markers specific to chromosome 1p. Loss of heterozygosity (LOH) was observed in 126 cases or 54% of the tumors. One marker, D1S496, at the 1p35 region showed the highest LOH, 28%. High frequencies of LOH were also detected by the markers, D1S488, D1S167 and D1S435, at the 1p31 region, 25%, 24% and 26% LOH, respectively. This suggests the presence of tumor suppressor genes at these two regions. Tumors with and without LOH at 1p were tested for association with clinico-pathological features of the tumors such as estrogen- and progesterone-receptor content (ER and PgR), age at diagnosis, tumor size, node status, histological type, S-phase fraction, ploidy, survival and LOH at chromosomes 3p, 6q, 9p, 11p, 11q, 13q, 16q, 17p and 17q. A significant association was found between LOH at chromosome 1p and high S-phase fraction and lower survival rate. Association was also found between LOH at 1p and chromosome regions 3p, 6q, 9p and 17q. A multivariate model including prognostic variables, showed that LOH at 1p is an independent prognostic variable and patients who have breast tumors with LOH at 1p have approximately a two-fold increase in relative risk of death. We conclude that screening for 1p deletions gives additional prognostic information that might be useful in breast cancer treatment.