Disease levels in common dab were found to be significantly lower in an area receiving sewage sludge in comparison to an adjacent control area; a ‘rested’ dump area with its adjacent control area; and a control area in a region well separated from sources of pollution. The pattern of serum lysozyme levels reflected the different prevalance of disease but kidney bacterial levels showed no discernible pattern between areas. Single pathological conditions examined in cod and haddock showed no trend. Until more information is available on the distribution and causes of natural variations in fish disease levels caution should be exercised in the interpretation of positive or negative correlations found between fish disease levels and pollution.