Information on prevalences of grossly visible diseases and parasites of flounder (Platichthys flesus) from the Baltic Sea is presented for 11 sampling areas on a transect from the Mecklenburg Eight to the Gulf of Finland. Among the 3008 flounder examined, highest overall prevalences were observed for lymphocystis (14.4%) and acute/healing stages of the skin ulcer disease (5.9%). The prevalences of liver neoplasms >2 mm in diameter (0.4%), skeletal deformities (0.6%), and fin rot/erosion (0.5%) were low. The only externally visible parasite recorded was Cryptocotyle concavum (28.2%). The results of a multivariate statistical analysis reveal that the diseases are influenced by a variety of host-specific (length, age, sex) and area-specific (salinity, temperature) factors as well as their interactions. By calculating the expected prevalence for a standardized fish population for each area and disease, the extent of area-specific effects not attributable to host-specific factors was estimated. (C) 1999 International Council for the Exploration of the Sea.