This study critically reviews recent data relating to the health effects of methylmercury in man and the attendant dose-response relationships. New data obtained from animal studies are also examined, including pre- and post-natal exposure. The consumption of fish represents the major source of methylmercury exposure in the general population. For this reason the mercury levels in fish throughout the world have been assessed, with particular attention being paid to the situation in the Mediterranean Sea. Here, there is limited knowledge of methylmercury intake in critically exposed populations such as fishermen, employees of the fish industries and their families.