The current-phase relation (CPR) of YBa2Cu3O7−x step-edge Josephson junctions was investigated experimentally at various temperatures. The investigated junctions were incorporated into a washer-shaped superconducting ring with inductance L ∼ 300 pH. The ring was inductively coupled to a tank circuit with a resonance frequency between 9 MHz and 40 MHz. The CPR was obtained from the measurement of the impedance of the phase-biased junction. At high temperatures and low critical current the experimentally observed CPR is sinusoidal. The same results were obtained at low temperatures when the rms flux noise in the ring is negligible. In the case of larger critical current and finite noise, deviations of the apparent CPR from sinusoidal dependence were found. The deviations can be described by thermal fluctuations of the magnetic flux in the ring. © 1998 American Institute of Physics.