We used activity diaries and snail detection to relate water contact and Schistosoma japonicum infection among a cohort of 178 residents on two islands in the Dongting Lake, China. Water exposure to each of 12 mapped water zones around the islands was calculated (m2 min/day) for each subject. Infected Oncomelania hupensis hupensis snails in this area are focal and were found in only five of the 12 zones, with the highest rate being 5.7%. Thirty- one subjects (17%) were re-infected with a mean intensity of 63.2 epg. Mean water contact was 7.9 m2 min/day; 98% of water exposure was due to economic activity and only 2% due to swimming or bathing, washing and other necessities of daily life. Males had more exposure and infection than females (P < 0.05). Infected subjects had more exposure (10.2 m2 min/day) than those not infected (7.44 m2 min/day) (P < 0.05). Compared with uninfected subjects, those infected had 2.9 times more exposure in infected-snail zones (P < 0.01). Also, human infection intensity (epg) correlated well with exposure to infected snail zones (r = 0.552, P < 0.01). People < 20 years old had the highest re-infection (21.4%) and intensity (3.77 epg). Median exposure for 20-49-year-olds (9.00 m2 min/day) was nearly double that of those aged < 20 or > 50 years old (5.5 m2 min/day). We conclude that map- referenced water contact and snail evaluation boosts accuracy of activity- diary measurements in large transmission foci for the Asian schistosome. Protecting against faecal contamination of snail inhabited sites, and against occupational exposure for island residents, should be a priority of future research. Potential strategies for migrating buffaloes and families living on visiting fishing boats are explored. (C) 2000 Elsevier Science B.V.