OBJECTIVE A prospective evaluation of the correlation between the serial clinical findings, serum cholinesterase levels, electrodiagnostic abnormalities and the daily atropine requirement was undertaken in 29 patients with confirmed acute organophosphate poisoning (OPP). RESULTS Clinical weakness conforming to the pattern found in 'Intermediate Syndrome' was noted in 19 patients (65.55%). It was associated with all types of organophosphate compounds and occurred in all patients in whom the serum cholinesterase on admission was less than 200 units. Three types of electrodiagnostic abnormalities were noted: single supramaximal electrical stimulus induced repetitive response, a decrement--increment response to 30 Hz repetitive nerve stimulation (RNS) and a decremental responses to 30 Hz RNS. The 30 Hz decremental response correlated best with the presence of clinically detectable weakness (sensitivity = 61.72%; specificity = 81.54%; positive predictive value = 73.91%; negative predictive value = 71.62%). Time trends evaluation revealed that the peak daily atropine dosages were given at a mean of 1.76 +/- 0.83 days in comparison to a mean nadir of serum cholinesterase of 2.48 +/- 1.97 days and a mean nadir of 9:1 ratio of 2.65 +/- 1.76 days. The 2-tailed correlation coefficient analysis and simple regression analysis revealed a positive correlation between serum cholinesterase levels and the 9:1 ratios (correlation coefficient: 0.59). A negative correlation was observed between the 9:1 ratios and the daily atropine requirement (correlation coefficient: -0.57) and between serum cholinesterase levels and daily atropine requirement (correlation coefficient: -0.49). CONCLUSIONS At admission, level of serum cholinesterase of less than 200 units is a predictor and the 30 Hz RNS decremental response could be a useful marker for the 'Intermediate Syndrome'.