The purpose of this study was to compare measurements taken after one and 24 hours of diagnostic occlusion in cases of basic and divergence excess types of intermittent exotropia. This was a prospective study of 43 patients (34 females) with 21 basic, 20 divergence excess and two convergence insufficiency types of intermittent exotropia – the latter two were excluded. Mean age was 13.4 ±7.6 years. Diagnostic occlusion was on the frequently deviating eye (19 patients) and on the suppressing eye (22 patients). Deviations angles (PD measures) for basic types at near fixation were 30.2 ±9.1 pre, 35.5 ±11.4 at one hour and 36.0 ±11.2 at 24 hours; and at distance fixation were 29.1 ±8.6 pre, 31.8 ±9.6 at one hour and 32.2 ±10.2 at 24 hours; with significant differences at both near and distance. Deviations angles (PD measures) for divergence excess types at near fixation were 29.9 ±12.4 pre, 26.0 ±14.9 at one hour and 28.6 ±14.7 at 24 hours; and at distance fixation were 35.6 ±9.1 pre, 35.4 ±4.5 at one hour and 35.5 ±10.9 at 24 hours; with significant differences at near. There was no significant differences from one versus 24 hours when applying a 5PD change to be clinically significant. The authors conclude one hour of diagnostic occlusion is as effective as 24 hours.