Idiopathic infantile nystagmus (IIN) consists of involuntary oscillations of the eyes. The familial form is most commonly X-linked. We recently found mutations in a novel gene "FRMD7" (Xq26.2), which provided an opportunity to investigate a genetically defined and homogeneous group of patients with nystagmus. We compared clinical features and eye movement recordings of 90 subjects with mutation in the gene ("FRMD7" group) to 48 subjects without mutations but with clinical IIN (non-"FRMD7" group). Fifty-eight female obligate carriers of the mutation were also investigated. The median visual acuity (VA) was 0.2 logMAR (Snellen equivalent 6/9) in both groups and most patients had good stereopsis. The prevalence of strabismus was also similar ("FRMD7": 7.8%, non-"FRMD7": 10%). The presence of anomalous head posture (AHP) was significantly higher in the non-"FRMD7" group (P less than 0.0001). The amplitude of nystagmus was more strongly dependant on the direction of gaze in the "FRMD7" group being lower at primary position (P less than 0.0001), compared to non-"FRMD7" group (P = 0.83). Pendular nystagmus waveforms were also more frequent in the "FRMD7" group (P = 0.003). Fifty-three percent of the obligate female carriers of an "FRMD7" mutation were clinically affected. The VA's in affected females were slightly better compared to affected males (P = 0.014). Subnormal optokinetic responses were found in a subgroup of obligate unaffected carriers, which may be interpreted as a sub-clinical manifestation. "FRMD7" is a major cause of X-linked IIN. Most clinical and eye movement characteristics were similar in the "FRMD7" group and non-"FRMD7" group with most patients having good VA and stereopsis and low incidence of strabismus. Fewer patients in the FRMD7 group had AHPs, their amplitude of nystagmus being lower in primary position. Our findings are helpful in the clinical identification of IIN and genetic counselling of nystagmus patients.