Calf hypertrophy is a typical clinical feature in neuromuscular diseases such as X-linked muscular dystrophies of Duchenne and Becker type and can be seen as an atypical feature in numerous other diseases. The diagnosis of calf hypertrophy usually is based on subjective visual assessment. The aim of this prospective study was to examine the prevalence of calf hypertrophy in a large number of patients with various neuromuscular diseases based on quantitative ultrasound measurement of calf muscle thickness. Additionally, true and pseudohypertrophy should be distinguished according to the absence or presence of abnormal muscle echointensities caused by infiltration of fat tissue. Fifty adult normal controls and 350 patients with various neuromuscular diseases were investigated. Absolute calf hypertrophy was diagnosed if the combined thickness of the gastrocnemius and soleus muscles exceeded the mean value of the control persons by at least 3.0 standard deviations (SD). Relative calf hypertrophy was diagnosed when the ratio of the combined thicknesses of the gastrocnemius and soleus muscles divided by the combined thicknesses of the rectus femoris and vastus intermedius muscles lay at least 3.0 SD below the mean value of the controls. Pseudohypertrophy was present if the echointensities of the gastrocnemius and soleus muscles reached or exceeded 3.0 SD above the mean value of the controls. An absolute hypertrophy of the calves was detected in 80 patients (= 22,9%; 64 true and 16 pseudohypertrophies), 16 patients exhibited a relative hypertrophy of the calves (= 4.6%; 12 true and 4 pseudohypertrophies). A significantly increased portion of both absolute calf hypertrophies and pseudohypertrophies as compared to the control group were found in juvenile proximal spinal muscular atrophy type 3, central core disease, centronuclear myopathy, benign X-linked muscular dystrophy of Becker type, autosomal recessive limb girdle muscular dystrophy, acid maltase deficiency, polymyositis, and granulomatous myositis. A significantly increased number of relative calf hypertrophies was present in juvenile proximal spinal muscular atrophy type 3, facioscapulohumeral muscular dystrophy, and inclusion body myositis. In the majority of the diseases included in the study, calf hypertrophy occurred in at least some patients. In conclusion, calf hypertrophy is a frequent and unspecific clinical feature in many neuromuscular diseases. Ultrasound is a convenient method for the exact definition of calf hypertrophy.