The aetiology of sporadic summer diarrhoea and enterotoxin (LT) antibody titres was studied among 43 adult patients in southern Brazil who had an acute, untreated diarrhoeal illness and 43 age- and sex-matched controls from the same area. A potential pathogen was identified in 33 of 34 patients and in 17 of 43 controls (p less than 0.01). 10 Shigella, one invasive Escherichia coli, one Salmonella and one Entamoeba histolytica were identified in 17 patients with inflammatory diarrhoea (faecal polymorphonuclear neutrophil leucocytes (PMN) present). In the other four only enterotoxigenic strains of E. coli were identified. Among 26 patients with non-inflammatory (no faecal PMN) diarrhoea, heat-stable (ST) toxin-producing coliforms were the most common pathogens isolated (27%). Heat-labile (LT) toxin-producing coliforms or serological rises to LT were seen in five patients, Strongyloides larvae in five, Giardia lamblia trophozoites in four and an LT-producing Salmonella in one. No pathogen was found in 10 patients with non-inflammatory diarrhoea. Among 43 controls, 11 LT coliforms, two Shigella and two Strongyloides larvae were found. 40 of 41 controls had detectable serum anti-LT antibody. A single ST-producing E. coli was found in one other control. No pathogen was identified in 26 controls. No rotaviruses or Norwalk-like viral agents were seen by direct or immune electron microscopy of stool filtrates. While LT-producing E. coli are frequently associated with endemic summer childhood diarrhoea in southern Brazil and are recognized causes of travellers' diarrhoea, this study shows that they are often carried asymptomatically by adults living in this region.(ABSTRACT TRUNCATED AT 250 WORDS)