The influence of lens parameter variation on the frequency and persistence of rigid gas-permeable (RGP) lens adherence after overnight lens wear was investigated. In 3 separate studies, 11 subjects wore lenses of different designs in each eye over 5 nights. No significant differences in the frequency of lens adherence were found with loose vs. tight peripheral fittings (49% vs. 42%), or with 9.6-mm vs. 8.7-mm diameter lenses (42% vs. 52%). However, a significantly higher frequency of adherence was found for flat-fitting lenses compared to steep-fitting lenses (84% vs. 49%; p < 0.05, Binomial test). Adherence was also significantly more persistent after eye opening with flat-fitting lenses, whereas steep-fitting lenses regained mobility more rapidly than the other lens designs tested. These findings suggest that the "suction cup" theory of RGP lens adherence does not adequately explain this phenomenon, and that the lens parameter variations investigated in this study do not reliably reduce or eliminate lens adherence during RGP extended wear.