PURPOSE Using a noncontact erbium (Er):yttrium--aluminium--garnet (YAG) laser, ablation of vitreous was compared to distilled water in vitro. METHODS The porcine vitreous body and distilled water were ablated in vitro at different pulse lengths and pulse energies. Selected pulse energies were 25, 35, 45, 75, and 100 mJ (pulse rate: 1 Hz; laser beam diameter at the surface of the sample: 2 mm). Pulse lengths were at 140 +/- 3 microsec, 190 +/- 4 microsec, and 240 +/- 5 microsec. The loss of weight in vitreous tissue and distilled water was measured using precision scales and corrected for evaporation, respectively. The Mann-Whitney U test was used to assess the significance of differences in ablation rates of water and vitreous. P < 0.05 was considered statistically significant. RESULTS Reproducible and constant ablation rates were found in both vitreous and distilled water in each of 10 consecutive series of 50 laser pulses at constant laser parameters. Ablation rates per pulse (microg/microsec) of vitreous tissue were as follows: 3.0 microg to 45.8 microg (140 microsec), 10.4 microg to 53.8 microg (190 microsec), and 17.9 microg to 24.2 microg (240 microsec). The ablation rates exhibited a linear correlation with increasing pulse energies and also with decreasing pulse lengths. Considering the pulse lengths of 190 microsec and 240 microsec with all pulse energies tested, the ablation rates of distilled water were significantly higher (P < 0.05) than ablation of vitreous tissue. The ablation rates at a pulse length of 140 microsec were not significantly different. The differences per pulse were as follows: 0.5 microg to 2.1 microg (140 microsec), 1.9 microg to 6.0 microg (190 microsec), and 3.5 microg to 8.7 microg (240 microsec). CONCLUSIONS Vitreous ablation is possible using Er:YAG laser. The ablation characteristics of vitreous have proved to be similar but not equal to that of water.