Heterophoria varies over time and asthenopic symptoms may be greater when the heterophoria is larger. Thus a close intra-individual correlation may exist between heterophoria and discomfort during reading. The authors investigated the variability of both parameters in 20 individuals who found prolonged reading strenuous – asthenopes. They also examined 20 non-symptomatic individuals. Associated phoria was measured by asking individuals to align monocular markers on the Mallett near vision unit. They were then asked to adjust Risley prisms so that viewing appeared most relaxed while they were looking at fully fusible pictures. Associated phoria and self-selected prism did not differ significantly between asthenopes and non-symptomatic individuals. The authors conclude that vergence position of rest does not have a causal relationship with asthenopic symptoms.