Latent/manifest latent nystagmus (LMLN) is a jerky type of nystagmus with the fast phase directed toward the fixating eye. A previous report described that the slow phase shows decreasing-velocity exponentials. However, it is sometimes difficult to differentiate between pure LMLN and latent nystagmus with congenital nystagmus. Furthermore, there has been no detailed report of quantitative analysis of the waveform of LMLN. Therefore, in the present study 18 cases with LMLN were clinically selected and their eye positions and/or strabismus were carefully studied. The eye movement was recorded by a photo-electric device. The patients fixated on a small white target placed at 0, 5, 10, and 15 degrees either from the center toward the right or toward the left in the horizontal plane. Eye movement in each position for at least one minute's duration was recorded on a floppy disc after being digitized by an A/D converter. Then, the time constant of the slow phases were estimated using the repetitive non-linear least square method by a personal computer (NEC, PC 9801). Details of the method have been described previously. The 18 cases were first classified into three groups, based on analysis of the slow phases; 1. decreasing-velocity type, 2. increasing-velocity type, and 3. combination of 1. and 2. In group 1. esotropia or intermittent esophoria was present in all cases (100%) and none of them had stereopsis. In group 2 on the other hand, exophoria or intermittent exotropia was present in 4 (51%) out of 7 cases, and 6 (96%) out of 7 cases had stereopsis.(ABSTRACT TRUNCATED AT 250 WORDS)