BACKGROUND To determine the prevalence of Orbscan II-derived keratoconus traits in relations of individuals with keratoconus and a control group and to apply these to a pedigree analysis. METHODS In a controlled, prospective, observational case series, four Orbscan II-derived corneal parameters were examined in relations of individuals with keratoconus and a control group of low myopes (<2.5 D). The four parameters and thresholds for abnormality (derived from a literature review) were as follows: average keratometry (> or =47.2 D), I-S value (> or =1.2 D), posterior float apex (> or =42 microm) and thinnest pachymetry (< or =463 microm). RESULTS Forty-four unrelated controls (88 eyes) and eight families with 90 members without known (178 eyes) and 11 members with keratoconus (19 eyes) were analysed. One of 88 (1.14%) control eyes had a single keratoconus trait, and none had more than one trait. Of 178 eyes from relatives of patients with keratoconus, 45 (25.3%) had one or more keratoconus traits. Relatives of patients with keratoconus had an elevated risk of possessing a keratoconus trait (relative risk 14.67, CI 2.07-104.07, P < 0.001) compared with controls. Approximately 53.3% of relatives with a keratoconus trait were evident on either pachymetric or posterior elevation indices alone. Six of eight families suggested dominant inheritance. CONCLUSIONS Keratoconus traits are common in relatives of patients with keratoconus. There prevalence may have been previously underestimated by using placido image-based topography alone where corneal pachymetry and posterior elevation are not assessed. This study suggests an autosomal dominant pattern of inheritance with variable expressivity in some families.