In this communication, the beam properties of one-dimensional leaky-wave antennas (1-D LWAs) are explored when the LWA scanning approaches endfire. By increasing the phase constant beyond the ordinary endfire condition, smaller beamwidths can be produced at the expense of increased sidelobe level (SLL). This tradeoff is quantified here for the first time for endfire 1-D LWAs. Simple CAD formulas are also proposed that describe accurately the beamwidth and SLL for 1-D LWAs operating at endfire. This communication is conducted for different radiation efficiencies covering most practical ranges. A closed-form expression for the beamwdith of a 1-D LWA with an infinite aperture is also given, in which case there are no sidelobes. These results allow for a better understanding and efficient design of LWAs radiating at endfire.