Flight experiments were conducted on an instrumented NASA-Langley 737-100 aircraft to investigate high-lift flow physics and to correlate and validate computational and wind-tunnel measurements. The possible reversion of turbulent attachment-line flow present at flight Reynolds numbers to a laminar state (relaminarization), under the action of strong favorable pressure gradients, has a potentially significant impact on the prediction of high-lift system performance from wind-tunnel tests and computational analyses. Boundary-layer state measurements, obtained in the most recent flight phase, taken around the slat and leading edge of the main element, are analyzed. Three transition processes on the slat (attachment-line transition, boundary-layer relaminarization, and subsequent retransition) are studied with and without a boundary-layer trip (trip belt) to vary attachment-line disturbance levels and to test the relaminarizing tendencies of the slat. Relaminarization still occurs in the presence of the trip belt. A separation bubble is revealed as the mode of retransition on the slat. The attachment-line Reynolds numbers at which natural and contamination-induced attachment-line transition occur in-flight are revealed. Relaminarization is also shown on the main element, indicating that relaminarization is not limited to the slat.