In a series of three idealized magnetohydrodynamic (MHD) calculations in cylindrical geometry, we show that an azimuthal magnetic field can produce well-collimated jets by the pinch effect. In a laser-produced plasma, the magnetic field is generated by crossed density and temperature gradients (∇n×∇T), and may be responsible for the ‘‘imprint problem.’’ In an astrophysical plasma, differential rotation associated with an accretion disk can generate the magnetic field, which may then be responsible for jet formation on stellar and galactic scales.