Muscle soreness that has a delayed onset is a common feature among both athletes and untrained individuals who engage in unusual exercises. This study was designed to provide additional morphological data to assess the relevance and significance of our previous findings that the sore muscles contain fibers with disorganized myofibrillar material. Muscle biopsies were obtained from 12 males (mean age 25 +/- 7 years), who suffered from severe soreness in their thigh muscles 18--72 h following eccentric bicycle exercise. Their strength performance were tested in parallel. Knee extensor strength was decreased at all angular velocities soon after exercise but gradually increased over the subsequent days although slower at the fastest contractions. Disturbances of the cross-striated band pattern were constantly observed. They originated from the myofibrillar Z-band, which showed marked streaming, broadening and, at places, total disruption. The disturbances were found in every second to every third fiber up to 3 days after exercise and in one tenth of the fibers 6 days following the exercise. Type 2 fibers were predominantly affected. Thus, the eccentric exercise gives rise to muscles soreness and influences, on mechanical basis and selectively with regard to fiber type, the fine structure of the contractile apparatus.