Rhabdomyosarcoma represents a large group of soft tissue sarcomas displaying heterogeneous histopathological features. In addition to their histopathological classification, the variable expression of a number of histopathological features may contribute to the heterogeneity and may be related to prognosis. Tissue sections of 113 well-documented, protocol-treated patients with long term follow-up (mean 6 years) were analysed by a panel of four paediatric pathologists. The following features were assessed: presence of rhabdomyoblasts, degree of maturation of rhabdomyoblasts, heterogeneous maturation patterns, mitotic activity, tumour necrosis, myxoid component, and septa. A scoring system was allocated to each index. High degree of maturation (amount of cytoplasm greater than surface area of the nucleus), absence of tumour necrosis (< 10% of tumour surface), and absence of septa (< 10% of tumour surface) significantly correlated with a favourable clinical course. Reproducibility in the assessment of these three features was good: mean kappa varying from 0.53 to 0.64. A rhabdomyosarcoma score function for survival was defined by: (-0.27 x degree of maturation score) + (0.007 x percentage septated area) + (0.031 x percentage tumour necrosis). Based on the score a two-grade system was elaborated, i.e. grade I (score < -0.20) v. grade II (score > or = -0.20). Rhabdomyosarcoma grade appeared to be the best factor in predicting patients survival: 69% long-term survival in patients with grade I v. 33% in patients with grade II (P = 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)