To study the variability of ciliary body lesions created by contact Nd:YAG cyclophotocoagulation and to evaluate modifications in probe design to reduce this variability.Contact transscleral Nd:YAG cyclophotocoagulation was performed on fresh, enucleated porcine eyes in three ways: using a standard, handheld fiber-optic probe (98 eyes); using the same probe with an adjunctive contact lens guide to control for probe pressure, angle, and position (69 eyes); and using a spring-loaded handpiece to control for probe pressure (148 eyes). Four laser lesions were created in each eye and were rated for size and severity of tissue response.For the three groups of eyes, the mean for size differences (largest lesion minus smallest lesion in millimeters for each eye) was 2.53, 1.65, and 2.36, respectively. The mean for severity differences (most severe lesions minus least severe lesion for each eye, based on a four-part subjective rating) was 1.9, 1.1, and 1.7, respectively. These measures of size difference and severity difference were significantly lower with the lens guide than with the other two systems (P = 0.022 and P = 0.020, respectively).These findings indicate that contact transscleral cyclophotocoagulation can be associated with considerable variation in the size and severity of the ciliary body reaction. This variation has a significant dependence on probe pressure and orientation against the eye and can be reduced by modification in probe design.