Gross external pathological abnormalities are frequently used as indicators of anthropogenic influence in estuarine and marine waters, but little baseline information is available to identify what constitutes normal prevalences (percentages of animals affected). To establish this baseline, trawl samples were collected from 120 randomly located estuarine sites in the Virginian biogeographic province (mid-Atlantic) in 1990 and from 220 randomly located sites in the Louisianian biogeographic province (Gulf Coast) in 1991 and 1992. Sediment contaminant concentrations were measured at each site. In all, 24,291 fish representing 143 species were examined for gross pathological abnormalities, and background prevalences were estimated to be 0.5% in midAtlantic and 0.7% in Gulf Coast estuaries. Fifteen types of gross abnormalities were noted. Skin lesions were the most prevalent abnormalities in both provinces, followed by ocular abnormalities (e.g., exophthalmia, keratitis) in the Virginian Province and branchial chamber abnormalities (e.g., parasites, gill arch deformities) in the Louisianian Province. The prevalence of gross abnormalities was about three times higher for demersal fish than for pelagic fish and was about eight times more prevalent at sites with high sediment contaminant concentrations. Among the major estuarine systems of the mid-Atlantic and Gulf of Mexico coastlines, pathological abnormalities were most prevalent in Galveston Bay and lowest in Long Island Sound and Mississippi Sound.