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Gaucher disease (GD) without primary neurological involvement (GD1) is now treatable with exogenous enzyme replacement therapy (ERT). ERT does not halt the fatal neurological progression of type 2 (infantile) disease (GD2), but in type 3 disease (GD3), neurological progression may be slowed down or possibly halted in some cases by higher doses of ERT. Therefore, it is crucial to distinguish between GD1 and GD3 disease for appropriate treatment. Saccade initiation failure (SIF) (ocular motor apraxia, supranuclear gaze palsy) is often the earliest neurological sign in GD3. This sign can be difficult to detect clinically, but is readily revealed as missed quick-phases during induced optokinetic and vestibular nystagmus. We investigated whether objective ocular motor assessment could improve the detection of GD3 disease. Eight children, diagnosed enzymatically with GD, were tested using D.C.-electro-oculography and/or video. In 6 children, optokinetic and vestibular nystagmus showed marked paucity of quick-phases making the eyes "lock up" at the limit of gaze, thus indicating SIF. In 3 cases the diagnosis was revised from GD1 to GD3. The diagnosis of GD3 was made in two children who were too ill for clinical assessment of SIF. Only 1 child had previous clinical evidence of SIF, and so GD3 was confirmed. One child was normal and the diagnosis of another remains uncertain. These results show that the possibility of SIF and hence GD3 may not be excluded by clinical examination alone. Thus we recommend that, where possible, objective eye movement assessment be carried out in children with GD.