The authors explore the pedigree analysis of familial cases of primary concomitant strabismus (PCS) to unravel the genetic determinants of this strabismus. Of 2301 cases of PCS, 39 (1.7%) had familial clustering. This study recruited 18 families with esotropia and 18 with exotropia. These had distinct phenotypes that did not overlap. Seventeen families with exotropia demonstrated vertical transmission while only 11 with esotropia had this. There was no specific general predilection in the distribution of the probands. In three families a combination of esotropia and exotropia was recorded.