To explain visual interpretation errors on angiograms, visual interpretation, caliper measurement, and computerized measure ment of cine film were compared using each of 10 graphic models and 10 acrylic models with "stenotic vessels". Stenosis >40% was overestimated and stenosis <40% underestimated by visual inter pretation. In caliper measurement, stenosis >40% at exposure of 90 kV was greatly overestimated by a degree similar to the estima tion by visual interpretation, and stenosis >40% at exposures of 74 kV and 58 kV was slightly overestimated. In computerized mea surement, the estimation was consistent with the actual degree of stenosis. Therefore, visual interpretation was not reliable for estimation, and computerized measurement was indispensable for estimation of vessel stenosis. Moreover, we consider the most com mon cause of error in visual interpretation to be optical illusions.

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