Paclitaxel and docetaxel are novel chemotherapeutic agents that promote the polymerization and inhibit the depolymerization of microtubules. Sensory neuropathy is common with these agents, particularly paclitaxel. We evaluated 64 patients treated with these drugs; 54 were followed prospectively. Eleven (17%, including six of the 54 prospectively followed patients) developed muscle weakness that was predominantly proximal. The weakness was idiosyncratic, occurring at any stage of treatment, had a variable course, and was reversible upon cessation of drug. All patients developed symptoms or signs of taxane-induced sensory neuropathy. Weakness was likely neuropathic in origin; electrodiagnostic studies suggested a distal axonopathy in some patients and proximal denervation (anterior horn cell or nerve root) in other.