OBJECTIVES: I. Determine the maximum tolerated dose and toxicity of oral fenretinide in patients with solid malignant tumors. II. Determine the pharmacokinetics of fenretinide and its metabolites. III. Determine the preliminary antitumor activity of fenretinide in this patient population. IV. Determine the recommended phase II starting dose of fenretinide. V. Determine whether fenretinide induces apoptosis in clinical specimens.