Twenty infants aged 25 days to 6 months who were consecutively investigated for apparent life-threatening events (ALTE) with negative results and 20 matched normal controls underwent an iontophoresis sweat test. A statistically significant elevated sweat potassium level (22.1 +/- 8.9 versus 12.4 +/- 6.5 mol/l) was noted in the ALTE patients compared with the control group (p < 0.001). No difference was found, however, between sweat sodium and chloride levels in the two groups. Na/K ratio in sweat was significantly different between the groups (p < 0.001). A between-groups discriminant analysis, using Na/K ratio as a discriminant variable, resulted in 80% accuracy in group assignment. A significant increase in sweat potassium concentration at night compared with day time was evident in ALTE patients. Elevated sweat potassium levels specifically characterized infants who experienced ALTE and may possibly indicate an underlying mechanism involving enhanced sympathetic activity.