The effect of virtual $\ensuremath{\Delta}(1236)$ states on the saturation properties of nuclear matter is studied within the framework of lowest-order Brueckner theory. The $\ensuremath{\Delta}$ is treated as a stable elementary particle. Transitions from the nucleon-nucleon ($\mathrm{NN}$) channel to the nucleon-$\ensuremath{\Delta} (N\ensuremath{\Delta})$ channel are caused by a nonrelativistic potential obtained from the static limit of meson theory. The coupled-channel potentials are constrained to fit the $\mathrm{NN}$ phase shifts. Saturation curves are calculated for the couplings $^{1}S_{0}(\mathrm{NN})\ensuremath{\leftrightarrows}^{5}D_{0}(N\ensuremath{\Delta})$ and $^{3}P_{1}(\mathrm{NN})\ensuremath{\leftrightarrows}^{5}P_{1}(N\ensuremath{\Delta})$, and the effects of other $N\ensuremath{\Delta}$ couplings to nucleon-nucleon $P$ and $D$ waves are estimated. Calculations are done using both the Reid soft-core and Ueda-Green potentials for $\mathrm{NN}$ partial waves not coupled to the $N\ensuremath{\Delta}$ channel. The $N\ensuremath{\Delta}$ coupling does not change the usual tendency of the calculated saturation points to lie in a narrow band in the energy-density plane that does not contain the empirical saturation point. This result is illuminated by a rough approximation to the Pauli and dispersion effects. We have also used this approximation to estimate the loss of binding due to $N\ensuremath{\Delta}$ coupling in those channels not treated by detailed calculation. Combining all our results, we find that at the empirical density (1) the inclusion of $N\ensuremath{\Delta}$ coupling in nucleon-nucleon $S$, $P$, and $D$ waves reduced the binding energy by about 3.3, 3.2, and 0.8 MeV, respectively, and (2) each particle spends about 3.7% of its time as a $\ensuremath{\Delta}$. All these figures vary roughly quadratically with the $\ensuremath{\pi}N\ensuremath{\Delta}$ coupling constant and increase rapidly with density. The size of the shift in energy depends strongly on the suppression of the short-range part of the two-body wave function, but our approximate formulas indicate that the tendency of the calculated saturation points to remain in a narrow band is independent of the short-range behavior of the two-body interaction, i.e., it is model-independent.NUCLEAR STRUCTURE Effect of $\ensuremath{\Delta}(1236)$ on saturation of nuclear matter studied in lowest-order Brueckner-Bethe-Goldstone theory.