To define and compare the magnetic resonance (MR) imaging findings of pelvic and thigh muscles in merosin-deficient and merosin-positive congenital muscular dystrophy, 10 patients with merosin-positive and six patients with merosin-deficient muscular dystrophy were examined in a 0.5 T MR imaging unit. Intensity and atrophy scores were given to individual muscles by two radiologists and were calculated for three muscle groups (pelvic, anterior thigh and posterior thigh muscles). Rectus femoris was affected less than the vastus muscles in 40 percent of cases in merosin-positive patients, whereas there was no selective sparing in merosin-deficient patients. Sartorius and gracilis were relatively spared in both groups. The most consistently affected muscles were gluteus maximus, adductor magnus and brevis in merosin-positive patients. Atrophy was more prominent in the adductor muscles in the merosin-deficient group. Intensity scores of anterior thigh muscles of the merosin-positive group were significantly higher than those of the merosin-deficient group (U = 8, p = 0.016). When stepwise logistic regression model was applied, intensity score of the anterior thigh muscles was found to be the best differentiating variable. The regression analysis model formed was able to differentiate the two forms with a sensitivity of 80 percent and specificity of 83 percent